Couple of hints:

• Can be done in O(n) -> single pass through data
• No division necessary and single multiplications by R are all that's needed
• Using map(C++) or dict(Java, Python) is a must -> can be unordered map (saves O(logN))
• Try to think forward when reading a value -> will this value form part of a triplet later?
• No need to consider (R == 1) as a corner case

Very interesting problem and it took a while to think it through properly. If anyone is desperate for the code, drop me a message.