Justin. Definitely something wrong with your Ariel box if you only have three gears. I think you will have to investigate. You can leave it in the frame, remove the clutch and pull the whole cluster out if needs be. But I would need to refresh my memory via my service sheets. Have you also checked that you have acceptable gearing regarding the sprockets? Ron
I'm a complete novice when it comes to these things but I can see I'm going to have to get the workshop manual out and have a look at the box. I've got a spare one so may have a look inside that one first.
I was hoping it might be a simple fix but I can see it might not be.