awesome.rb

#======= controller

def new 
    @model_a = ModelA.new
    @model_a_param = ModelAParam.new
end

def create
    @model_a_param = params[:model_a_param] # strong_parameterは省略ね
    
    if @model_a_param.invalid?
        render action: new
        return
    end
    
    # ...
    # ... model_a_param をごにょごにょ
    # ...
end

#======== _form.html.erb

<%= form_for @model_a_param do |f|
    <%= f.input :attr_a %>
    <%= f.input :attr_b %>
    <%= f.input :attr_c %>
    
    <!-- ... submit button etc-->
<% end %>


#======== RAILS_ROOT/app/params/model_a.rb とか

class ModelAParam << BaseParamModel 
    validates_presence_of :param_a, :param_b, :param_c
    attr_accessor :param_a, :param_b, :param_c
end

#======== ちなみに
class BaseParamModel
  include ActiveModel::Validations
  include ActiveModel::Conversion
  extend ActiveModel::Naming

  def initialize(attributes = {})
    attributes.each do |name, value|
      send("#{name}=", value)
    end
  end

  def persisted?
    false
  end
end