The piranha will continue to grow after you move them if given clean water, and a enhanced diet.
135 for three Piranha? Thats quite excessive.
A 55 is around a minimum (especially since the tank is but 12 inches front to back), but you should save till then and budget for finding a 65 or 75, decorate wisely, and add additional filtration to handle large feedings.A school of three is a bit low, but you're not going to be able to house anything like the population a wildschool would be anyways. Consider 20 gallons per piranha, as a minimum, with strong in-tank flow, and significant filtration.
Keep low watt bulbs for dim lighting over the tank with some low light plants. Bamboo, mosses, cryptocoreyne, java fern... These will eat the high nitrates. Your filters must handle the solide wastes, and good flow inside the tank from additional powerheads will ensure that waste goes to the filter and does not collect under a log or in a dead flow zone inside the tank.