The second strap is superflupous and often it will cause an unsightly pull across the front apron of the kilt. The top one is the only functional strap, so why bother with an additional, unnecessary strap that may just make the front of the kilt look funny?

Having said that, all my kilts have three straps, but the next custom kilt I order will have only two.

But of course, if you like the look of three, put on three. I read in Scotty Thompson's book that he put the third one on the left to hold the under apron in place, and only put one of the right side. How's that for a switch?