yeah, you'd need more "floor" space than anything else...a standard 55 for 3 adult sengals is too small, but maybe for 3 juvenilles...I'd say ATLEAST a 75 for adults...
Senegal doesn't grow much bigger than 12-in, and they grow at slow rate, so a 40-gal (36"x15"x17") should be sufficient for 2-3 of them. 29-gal should be the minimum tank size