You could even use something as small as a 6qt as well: the snugger the enclosure and hides the better for babies (though I too personally like the 15 qts for the younger ones)
Please separate the three you have and house them all and any others you might acquire separately: having them all together currently is likely really stressing them out. There's many threads on here that tell how to properly set up a tub setup if you're new to snake-keeping in tubs.