base 3 is a positional numeral system with three as its base. It uses 3 different digits for representing numbers. The digits for base 3 could be 0, 1, and 2.

base 20 is a positional numeral system with twenty as its base. It uses 20 different digits for representing numbers. The digits for base 20 could be 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, a, b, c, d, e, f, g, h, i, and j.