Fixed drives come in 1/8 and 3/32 and the 1/8 cog is actually thicker as are the chain rings to add strength and also to reduce wear... there is no cog spacing on a monospeed bike to worry about.

Most English 3 speeds like a Sturmey Archer typically use 1/8 drives while Shimano 3 speeds use a 3/32 drive... coaster bikes typically use 1/8 drives.