It might be. There were two main types of three speed. The standard one and a heavy duity one. If the 3 speed is original to your I6, then no, it's not. You could use it on a V8 if you wanted to and probably not run into any problems provided that:
A)it's a very mild small block
B)you don't constantly roll on the throttle
the six cyl tranny isnt even close to the V8 tranny, at least not untill 68. if you try using the six cyl tranny behind a V8 you will shred it in short order.