When the murderers come across Banquo
When the murderers come across Banquo. The third murderer recognizes him and says ‘Tis he.” which means he knows his face. Interestingly enough, also Banquo recognizes him and he doesn’t think of them as a threat. He then says ”It will be rain tonight.” I think he says this in order to greet them and show them that he recognizes them. I think who he saw is not a close friend like a thane or a nobleman but also, he definitely saw someone who he knows. I think this behaviour pattern seems like he is talking to a servant. Moreover, if he didn’t know the third murderer’s face he could’ve attacked them as well. I think the second-best soldier in the country is able to fight with three murderers.
Later on, the scene, when the murderers attack Banquo, he shouts ” O treachery!” This shows he thinks this a betrayal if he didn’t know who the murderers worked for he wouldn’t say that. I also think he knows that the murderers have a bond with Macbeth. Which the third murderer aka Servant has.
I also think that it is the third murderer is the servant because the other possible third murderers are all in the Macbeth’s party. The servant is the only one who is not supposed to be there.