How would you make a 3-bit binary adder that adds two instead of 1?
Note that when adding 2 to a binary number, the low bit doesn't change.  You therefore simplify the problem to pass the low bit, and a 2-bit adder you add one to for the upper bits.  Yes, it really is that easy.

By the way, if you're only ever going to add 1 with a adder, then it's generally called a "counter" instead of a "adder".