Three 12 inch woofers do not have a radiating area equivalent to 36 inches (3*12). Two of the Emotivas would only have 28% more radiating area than a single HSU VTF-15H. If your math worked it would be over 2.5 times more radiating area which I admit would be pretty awesome, sadly not possible.
Regardless, you're comparing wattage on a ported 15 inch subwoofer to a sealed 12 inch? Apples to oranges, there's more to consider than just wattage, especially between those configurations.
Would 3 Emos beat the 15 inch HSU? I don't know, to be honest the HSU would probably play lower, however you'd have better control over room response with three subs than the one HSU. I'd personally go for two subs like the SVS PB10 or something similar, staying with ported design instead of sealed unless absolutely necessary (and I have a new sealed sub in my stereo system).
To the OP, with that size space and your budget I'd get multiples of a smaller ported sub rather than either option you asked about.