Looking for live quantity update solution

Hello

I am looking for a solution that will allow customer to to “example” choose 3 small, 4 large & 5 X Large and have the main quantity field update to 12.

Not sure if I need a plugin for this or if it can be done with some easy to implement JavaScript.
Sample page https://www.elsographics.com/product/american-backcountry-vapor-basic-t/

Thanks for any advice or direction here
Nelson

The page I need help with: [log in to see the link]