I am not sure if I read that the way you meant it but if you are in third gear and need to go a bit slower you should not just pull in the clutch and coast... that will wear out your clutch really fast.... you can close the throttle in any gear and the engine will slow you down (engine breaking).

as for the rear wheel sliding when you break... it is probably a heavy right foot like everyone said but you might want to double check that the rear wheel is alligned properly.