Presumably you mean cup as in the standard unit of measurement? In which case 16 US cups are equal to 1 US gallon, so times that by three. Or do what WRW recommends instead so you don’t have to lose count of how many cups you take out!
Having to keep count of 48 cups will become... tedious. You could get a bigger container and measure how many cups that are in that to make it easier.
Generally for my tanks I'll do the measured cups/gallon thing the first time and leave inconspicuous marks on both sides of the tank for reference. From there it's Syphon City. Won't even have to keep track as you'll know how far you need to place the tubing.