I think he can beat 2 together. Not sure about 3, mainly because we don't know much about Trebol. In latest chapter he apparently used a move that one shoted Law(although extremely tired, injured and exhausted), so he may be stronger than i think. Diamante's most powerful move is a lower version of 360pound cannon and it even requires him prep time. In swordsmanship he is so terribly outclassed that he couldn't hold a candle if Zoro were to engage him seriously.
It took him a while to defeat Pica straight up because of his instantaneous rock travelling ability, but the other top executive are wide open, so all what he need to do is to prioritize on them first and finish Pica later. He already showed he could ignore Pica, dodging his attacks while discussing with Robin some chapters back, so it shouldn't be that difficult especially with his superior haki. It would depend on Trebol though