From memory of a few years ago, so might be a bit fuzzy: When I bought the boat, the Spinlock triple clutch lever axle was seized and opening one opened all three. I tapped the axle out, which took a lot of tapping as it was jammed in tight. There was no corrosion visible, seemed like some part had swelled up? I used the Dremel with a suitable small tool (can't remember which) to clean up and minimally enlarge the holes in the levers. It has been fine ever since.
I think you can see the end of axle on the side of the clutch. Put a mark on it and then see if it turns. The axle shouldn't be turning in the clutch body, instead the lever should be turning on the axle. If it does turn, I would suggest removing the axle and doing as above.