High Ground’s effect is cumulative. If you have a creature that can already block an additional creature, now it can block three creatures.

High Ground allows you to make some complicated blocks. For example, if you’re being attacked by three creatures (call them A, B, and C) and you control three creatures (X, Y, and Z), you can have X block A and B, Y block B and C, and Z block just C, among many other possible options. The defending player chooses how each blocking creature’s combat damage will be divided among the creatures it’s blocking.