Need a few more details. 3 ft space, is that width or length? What is the other dimension? How far are the joists spanning? Is the tank parallel or perpendicular to the joists? Is this a house, apt, new, old. Is there any additional support like a pillar below? Are you positioning it closer to a corner or center of the room? If the room is big, it will have a center beam that the joists connect to. Given no other info, I would say the floor can most likely support 46 gals.