Kakashi is a very careful fighter especially against enemies unknown to him, he often try to learn first the abilities of his opponents and look for weakpoints to exploit. He is a precision type, so he always choose to bring down his opponents with minimal toll. Apparently his strength is also his weakness. His reluctance to pull his biggest gun right off the bat has often led him to extended fights which is very dangerous for him because of the chakra drain just by using sharingan, leaving him empty when the time he truly needs it (against pain and kakuzu). For him to win against the sannin 1 on 1, he must avoid prolonged battle. We know that sometimes Kakashi is willing to finish an enemy off using his strongest techniques when he recognizes threat (ex. Ororchimaru, Deidara, Tobi, Gedo Mazo, Juubi).
So the real question is, does he think the sannin are great threat, cause if he thinks so, we know he would try to use kamui on spot. None of the sannin have showed any defence against kamui. It's fast and an all range attack and able to warp huge mass in exchange large amount of his chakra. And Kakashi is very precise at using it like a sniper, he is able to warp fast moving objects as have been shown countless times in the manga. The only person countered it so far is obito.
We know kakashi has the capabilty to finish any of the sannin with his strongest jutsu. Do I think he's above their level? No. But I think he is on their level. He has the potential to surpass them, as we can see he has improved in this war. He has the skills to go toe to toe against any of the three but not for extended period of time, each of the sannin is chakra monster, so his best bet is to finish them off quickly.
