3 in a 2x4 should be just fine. It will depend on the strains and how big you let them get. You will invariably overshoot or undershoot in the process of learning what works best in your setup. Sometimes growing different strains in the same space can give you headaches, but I do it all the time for the variety. The stretch will really have two components. One is genetics, the other is your lighting. If your lighting is dialed in, you can keep them from stretching any more than their genetics want them to. If they are searching for light, they will tend to stretch more to find it. That said, a lot of strains that have a lot of sativa or OG in the wood pile can have a major stretch no matter what you do with your lighting. Some just be that way. Cheers