All Paul was saying was that what Jesus said at Mt 18:16, (see also De 19:15) applied to that congregation. The tie-in is not how many times he attempted to see them, but that there were matters that may still need judicial attention within the congregation and that rule must be followed. He just got done writing:
"For I am afraid that somehow when I arrive, I may not find you as I wish and I may not be as you wish, but instead, there may be strife, jealousy, outbursts of anger, dissension, backbiting, whispering, (or "gossip.") being puffed up with pride, and disorder. Perhaps when I come again, my God might humiliate me before you, and I may have to mourn over many of those who previously sinned but have not repented of their uncleanness and sexual immorality and brazen conduct (or "shameless conduct.") that they had practiced."
- 2 Cor 12:20,21
Paul did not actually succeed in arriving in Corinth 3 times. He actually visited the Corinthians twice. After his first visit, at which time he established the congregation, he made a plan for a second visit, which failed. But “the third time” that he planned or got “ready” he was successful, for he was able to see them again in about 56 C.E. (2Co 1:15; 12:14; 13:1) So he was not applying himself to being witness 1, 2 and 3 from 3 separate occasions.
All he was reminding them was to not overstep the rule and that he was attempting to visit them again. If there were matters still that needed addressed, upon arriving he would see to it that they got handled - provided the witnesses were presentable.