It's not that difficult really. A combination lock works using spaces - the three number combination means there are three spaces. If you can work out the relationship between the numbers in the outer ring and those in the inner ring, those spaces should be easy to spot.
Looks like a broad hint is needed. All of the numbers in the outer ring can made using the numbers in the inner ring - except for three.
Sorry - had forgotten about this... Pair any two numbers in the inner circle then look for the answer you get in the outer circle. If it's there, cross it off. Repeat that until there are three numbers in the outer circle left uncrossed - those are the answer.