Hello. Can anyone please help me with this problem? My bike should have three gears but only has two. When i try to switch to the first gear it goes into the third gear again. It is a peugot, thirty years old with internal hub gears. The wheel is new though. Regina is the make.

Is this problem simple to repair and if so how do i go about it? (If it's tricky i will bring it to a bike mechanic.)