When I grow with T5's I keep my plants an inch or so away. I do not let the plants get to tall because T5's have a hard time penetrating anything more than 6 or so inches away. In my experience I would get colas a few inches long and the rest of the plant would be like popcorn. In my opinion a 3 foot plant is a bit big for T5's.
edit: for your next grow I would build a screen the size of your T5 fixture and do a scrog grow.
this is all my opinion other people might not agree.