The T-100 was originally conceived with three turrets and was eventually built with two. It was in competition with a similar design - the SMK - but neither were adopted and instead a single turret version of the SMK was ordered as the KV-1.
The T-100 started with three turrets but ended up with two. Similarly, the SMK also had three turrets initially and then became a single turret design for the KV-1.