Agreed. The minimum port is 1/4 cone area; that's absolute minimum, which is 132 in^2 for 3 15's. I'd also tune quite a bit higher. 38hz tuning is going to kill from 15-40hz, but you'll have very little above 45hz. I'd start with a 45hz tuning if you like the lows, 50hz if you're looking for more 50-70hz output. A good starting point would be 6cf sealed, 12cf ported with 3 8" aeros or even better, two 10" aeros if you can find them. Again, this is just a good starting point. You'll probably have to play around with sealed/ported volume and tuning to get it perfect.
Edit: Based on your dimensions, you don't have enough room for three 15's in a 4th order. Could get away with two 15's though or three 12's.