Mike I can understand. Gustavo knows that Lalo needs bail and that he will likely use his lawyer to get it. So he just had Mike follow Saul. Comes with the bonus of getting back the money that the DEA had gotten from him thanks to Lalo and then some interest and keeps Lalo in jail.
But I really can't see a third party. The territories are claimed and there is no war going on aside from the obvious one. So either they were Gustavo's men and just there to make it look like to Saul like he was attacked, so he sells it the right way to Lalo when he comes back without the money, or they were Lalo's men that were told to go there by Nacho on orders of Gustavo, or they were there on orders of Lalo and really supposed to off Saul - which doesn't make a lot of sense, because Saul is scared to death by Lalo. He owns him. The more I think about it, the more I think it could actually be Gustavo and his men - to get Saul be grateful for saving his life and own him like he owns Nacho. But Lalo seemed quite relaxed when he was confronted with Saul missing, so we'll see I guess.