Anything can be done, but in this case there are a lot of reasons to hedge that bet. The parent species of flowerhorns are not known for being particularly easy to keep with others. How much space is an issue, changes with maturity, and if a pair decides to try to breed all bets are off for the rest of the fish. As a blanket statement there are lots of reasons it would fail if I simply said sure it can be done.
For example, something 3 gallons can be called an aquarium, so in that case of course not.