I have a 3 bulb (60W each) ceiling fixture. It has been working for 30 years without a problem. It suddenly stopped working. The breaker was not tripped. The switch is fine.
I changed 2 of the 3 sockets and it began working fine. However after a while it just went dead again. Prior to this I could leave this light for 24 hrs without any problem.
Any ideas would be appreciated.

Thanks

Based on the opportunity cost of the time you are spending, and the likely replacement
Cost of a new fixture? I would expect trying to repair this fixture to be a more expensive endeavor...
Of course if all that doesn't matter and you would like to repair this fixture, I would try replacing "all" the wiring, sockets and electrical parts within. Using a continuity tester could also be helpful to help you pinpoint a problem area.