Sahil has given you the solution to your problem. What you want to do is, as he said, shrink the other columns, not grow just increase the width of the third column.
By the way, you probably don't want to use GSAP just to add classes to your elements. You can toggle that without GSAP - As a rule of thumb it's actually better to animate either with CSS transitions/animations or with GSAP, not both. They'll be fighting for control and causing all sorts of headaches.
It really looks like you don't even need GSAP for what you are trying to achieve, simply a :hover pseudo attribute and some transitions and classes.