If IW5M I don't really know, except the fact that class are locked because some weapons/attachment are forbiden (like GL, grip, G36C etc)
I don't know if you can use your own classes.
On TeknoMW3, it's the same stuff.
But with something more.
If you are on a server with default classes on the server (Like iSnipes servers)
You can use NH9's triple threat to unlock the 5 basical MW3 classes instead of the snipes/server default ones.
EDIT : I guess some people can make a tutorial to use custom class as the 5 basicals ones too xD
But I don't think they will