OK, I'm guessing here, but it sounds like the glowplugs are acting normally. 20Ax12V is about 240 watts, tapering to 10Ax12v or 120 watts. That should make a nice, hot glow!

6x10A or 60A total current is not too much draw; I think I remember an 80A 'master' fuse link in the relay. Each relay contact is seeing 20A at start and 10A long-term, and shouldn't be enough to burn a contact.

If the glow plugs are normal, I'd have to suspect something in the lead wire from the relay to the plug, some kind of intermittent short. Those can be the devil himself to troubleshoot; look for rub spots on #3 wire?

Oh, one last thought. Was the relay replaced, or did the old relay (with #3 contact burned out) somehow get plugged back in, leading you to think it burned again? That's fishing, I know, but it is possible.

Somewhere there has to be an explanation. "The Truth is Out There."

Good luck, and good hunting.