One of the problems you are going to have is that a 3' fixture is almost exclusively specific to the aquarium or reptile industry.
There are a ton of great fixtures that can be had at a great price in either 2' or 4', but 3' is just not widely used outside of our hobby, or at least a very narrow use as compared to the general production of the other two.
Agreed. The other problem with non-specific fixtures is cheap ballasts that fail under the load of "our" bulbs. After trying to use various cheaper "shop light" fixtures I finally stuck with aquarium hoods. They lasted a lot longer and were repairable.