Example for how to swap multiple tokens in one on Ropsten

MultiSwap.sol

// SPDX-License-Identifier: MIT
pragma solidity =0.7.6;
pragma abicoder v2;

import "https://github.com/Uniswap/uniswap-v3-periphery/blob/main/contracts/interfaces/ISwapRouter.sol";
import "https://github.com/Uniswap/uniswap-v3-periphery/blob/main/contracts/interfaces/IQuoter.sol";

import {IERC20, SafeERC20} from "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/release-v3.4-solc-0.7/contracts/token/ERC20/SafeERC20.sol";

interface IUniswapRouter is ISwapRouter {
    function refundETH() external payable;
}

interface IUniswapV2Router02 {
    function swapExactTokensForTokens(
        uint amountIn,
        uint amountOutMin,
        address[] calldata path,
        address to,
        uint deadline
    ) external returns (uint[] memory amounts);
    function swapTokensForExactTokens(
        uint amountOut,
        uint amountInMax,
        address[] calldata path,
        address to,
        uint deadline
    ) external returns (uint[] memory amounts);
    function swapExactETHForTokens(uint amountOutMin, address[] calldata path, address to, uint deadline)
        external
        payable
        returns (uint[] memory amounts);
    function swapTokensForExactETH(uint amountOut, uint amountInMax, address[] calldata path, address to, uint deadline)
        external
        returns (uint[] memory amounts);
    function swapExactTokensForETH(uint amountIn, uint amountOutMin, address[] calldata path, address to, uint deadline)
        external
        returns (uint[] memory amounts);
    function swapETHForExactTokens(uint amountOut, address[] calldata path, address to, uint deadline)
        external
        payable
        returns (uint[] memory amounts);
}

// https://docs.bancor.network/developer-quick-start/trading-with-bancor#trading-from-your-smart-contract
// https://app.bancor.network/eth/swap?from=0xEeeeeEeeeEeEeeEeEeEeeEEEeeeeEeeeeeeeEEeE&to=0xF35cCfbcE1228014F66809EDaFCDB836BFE388f5
// https://ropsten.etherscan.io/tx/0x21b95960b1a7c832c91e705390420edf3faa35b18469a8bc517056d88af9634e
interface IBancorNetwork {
    function convertByPath(
        address[] memory _path, 
        uint256 _amount, 
        uint256 _minReturn, 
        address _beneficiary, 
        address _affiliateAccount, 
        uint256 _affiliateFee
    ) external payable returns (uint256);

    function rateByPath(
        address[] memory _path, 
        uint256 _amount
    ) external view returns (uint256);
}

// sushi https://ropsten.etherscan.io/tx/0x727301c32fcdbb29e14203610b26c7ab7f44f5d940057c2c39ecc0ae9e919c0f
// https://app.sushi.com/swap?inputCurrency=0x9108Ab1bb7D054a3C1Cd62329668536f925397e5&outputCurrency=0xF35cCfbcE1228014F66809EDaFCDB836BFE388f5

// uni https://ropsten.etherscan.io/tx/0xc23e6efa4c95747cb1421b582b1d29ce1ae1a529f84c28a94f74536997358262
// https://app.uniswap.org/#/swap

contract MultiTrade {
    using SafeERC20 for IERC20;
    
  // Bancor
  IBancorNetwork private constant bancorNetwork = IBancorNetwork(0xb3fa5DcF7506D146485856439eb5e401E0796B5D);
  address private constant BANCOR_ETH_ADDRESS = 0xEeeeeEeeeEeEeeEeEeEeeEEEeeeeEeeeeeeeEEeE;
  address private constant BANCOR_ETHBNT_POOL = 0x1aCE5DD13Ba14CA42695A905526f2ec366720b13;
  address private constant BNT = 0xF35cCfbcE1228014F66809EDaFCDB836BFE388f5;
  
  // SushiSwap
  IUniswapV2Router02 private constant sushiRouter = IUniswapV2Router02(0x1b02dA8Cb0d097eB8D57A175b88c7D8b47997506);
  address private constant INJ = 0x9108Ab1bb7D054a3C1Cd62329668536f925397e5;
  
  // Uniswap
  IUniswapRouter private constant uniswapRouter = IUniswapRouter(0xE592427A0AEce92De3Edee1F18E0157C05861564);
  address private constant DAI = 0xaD6D458402F60fD3Bd25163575031ACDce07538D;
  
  constructor() {
      IERC20(BNT).safeApprove(address(sushiRouter), type(uint256).max);
      IERC20(INJ).safeApprove(address(uniswapRouter), type(uint256).max);
  }
  
function _tradeOnBancor(uint256 amountIn, uint256 amountOutMin) private {
  bancorNetwork.convertByPath{value: msg.value}(_getPathForBancor(), amountIn, amountOutMin, address(0), address(0), 0);
}
  
function _getPathForBancor() private pure returns (address[] memory) {
    address[] memory path = new address[](3);
    path[0] = BANCOR_ETH_ADDRESS;
    path[1] = BANCOR_ETHBNT_POOL;
    path[2] = BNT;
    
    return path;
}
 
  
function _tradeOnSushi(uint256 amountIn, uint256 amountOutMin, uint256 deadline) private {
    address recipient = address(this);
      
    sushiRouter.swapExactTokensForTokens(
        amountIn,
        amountOutMin,
        _getPathForSushiSwap(),
        recipient,
        deadline
    );
}

function _getPathForSushiSwap() private pure returns (address[] memory) {
    address[] memory path = new address[](2);
    path[0] = BNT;
    path[1] = INJ;
    
    return path;
}

function _tradeOnUniswap(uint256 amountIn, uint256 amountOutMin, uint256 deadline) private {
    address tokenIn = INJ;
    address tokenOut = DAI;
    uint24 fee = 3000;
    address recipient = msg.sender;
    uint160 sqrtPriceLimitX96 = 0;
    
    ISwapRouter.ExactInputSingleParams memory params = ISwapRouter.ExactInputSingleParams(
        tokenIn,
        tokenOut,
        fee,
        recipient,
        deadline,
        amountIn,
        amountOutMin,
        sqrtPriceLimitX96
    );
    
    uniswapRouter.exactInputSingle(params);
    uniswapRouter.refundETH();
    
    // refund leftover ETH to user
    (bool success,) = msg.sender.call{ value: address(this).balance }("");
    require(success, "refund failed");
}
  
// meant to be called as view function
function multiSwapPreview() external payable returns(uint256) {
    uint256 daiBalanceUserBeforeTrade = IERC20(DAI).balanceOf(msg.sender);
    uint256 deadline = block.timestamp + 300;
    
    uint256 amountOutMinBancor = 1;
    uint256 amountOutMinSushiSwap = 1;
    uint256 amountOutMinUniswap = 1;
    
    _tradeOnBancor(msg.value, amountOutMinBancor);
    _tradeOnSushi(IERC20(BNT).balanceOf(address(this)), amountOutMinSushiSwap, deadline);
    _tradeOnUniswap(IERC20(INJ).balanceOf(address(this)), amountOutMinUniswap, deadline);
    
    uint256 daiBalanceUserAfterTrade = IERC20(DAI).balanceOf(msg.sender);
    return daiBalanceUserAfterTrade - daiBalanceUserBeforeTrade;
}
  
function multiSwap(uint256 deadline, uint256 amountOutMinBancor, uint256 amountOutMinSushiSwap, uint256 amountOutMinUniswap) external payable {
    _tradeOnBancor(msg.value, amountOutMinBancor);
    _tradeOnSushi(IERC20(BNT).balanceOf(address(this)), amountOutMinSushiSwap, deadline);
    _tradeOnUniswap(IERC20(INJ).balanceOf(address(this)), amountOutMinUniswap, deadline);
}
  
  // important to receive ETH
  receive() payable external {}
}

I was able to successfully compile the code, but I don't think you can trade on Bancor in the US (which is where I'm from), so I tried contacting soliditydeveloper.com what changes I make to the code five days ago, but I did not receive a response from them.

Hey @ThePlankton5165, the code is specific to Bancor and won't work with something else. However it's a smart contract, so you should be able to use it from the US without problems. Only the frontend from Bancor won't work.