I have 3 GTX 1060 6 gb gpu's of the same brand on one rig.
But there is one of them that mines considerably faster than the other 2, although they are all overclocked the same way...
I don't understand why there is one almost 25% faster than the other 2?
No psychics here I'm afraid, you need to tell us what you're mining. If it's Ethereum (or any other Dagger coin) then this huge 25% difference could be due to the memory timings: one of your 1060s could have Samsung chips (with tighter timings) and the other two could have Hynix chips (with looser timings). Easy to check the memory brand by running GPU-Z. If it's not Ethereum though then it won't be as easy to account for the 25% difference.