Hi,
I have made a form that must retain 3 columns regardless of device size. The max i would ever want it to be is 600px on a desktop. Does anyone know how i wuld change the CSS to use 100% width for all devices with a max of 600?

Have you tried using an unresponsive form?
Pick the default non responsive theme form the theme selector and it will allow you to set you forms fixed width.