This will be followed by the CreateSnapShot lambda function. As CreateSnapShot will keep on making the backups( if you have it scheduled for everyday or every month) but won't delete it. This function will delete the snapshot and keep only the latest 3 copies of the snapshot

PruneSnapShots.py

from datetime import datetime
import boto3

def lambda_handler(event, context):
    account_id = boto3.client('sts').get_caller_identity().get('Account')
    ec2 = boto3.client('ec2')
    regions = [region['RegionName']
               for region in ec2.describe_regions()['Regions']]
                
    #Iterate over each Region
    for region in regions:
        print("Regions", region)
        ec2 = boto3.client('ec2', region_name=region)
        
        response = ec2.describe_snapshots(OwnerIds=[account_id])
        snapshots = response["Snapshots"]

        # Sort snapshots by date ascending 
        snapshots.sort(key=lambda x: x["StartTime"])

# Remove snapshots we want to keep (i.e. 3 most recent)

        snapshots = snapshots[:-3]

        for snapshot in snapshots: 
            id = snapshot['SnapshotId']
            try:

                print("Deleting snapshot:", id)
                ec2.delete_snapshot(SnapshotId=id)

            except Exception as e: 
                if 'InvalidSnapshot. InUse' in e.message:

                    print("Snapshot {} in use, skipping.".format(id))
                    continue