I am a newbie at this, but I think the general rule of thumb on tank height is 3X max adult height of the searhorse species being kept. With the added effort of supplying BBS to dwarf horses and the potential water fouling, it'd be iffy, especially if you hadn't kept any seahorses before. if you know someone who is keeping three in a 2 gallon, get some info. What kind are they, how long has he had them, how much experience with seahorses, etc.
They are really cool to watch, but with numerous feedings every day and the waste therein, since I've been keeping them, the hobby has gotten much more demanding (still have everything I've always had, just thought I needed another challenge;)). I never fed my reef fish very often, so going to a dedicated 3X daily for the adults has been an increase in work load. Next, the addiction leads me to phyto cultures, which will only further demand more time. Definitely a labor of love (and an ongoign commitment I didn't fully anticpate going in)...