SunRace Sturmey-Archer S-RF3 hubs are lubricated from the factory with Sturmey SA103A synthetic grease. I would NOT add oil to this synthetic grease. I would never put 3 in One in an IGH in any case - it is both too acidic and it has a vegetable based component that will go rancid and turn into a very non-lubricant.

If you really think it might be a lubrication problem, open the hub and check. (I bet you'll find the internals are slathered in SA103A.)

The S-RF3 manual is located here. There is a fault diagnosis table on page 3.

HTH,
tcs