During the finale of The Good, The Bad And The Ugly it was clear that Blondie was always going to shoot Angel Eyes, given his prior knowledge that Tuco's gun was unloaded. It was also clear that Tuco was going to shoot Angel Eyes because of his prior affiliation with Blondie.
I've watched the film numerous times, but can't make out who Angel Eyes was going to shoot.
It would make sense for him to shoot Tuco given that Blondie might not have written anything on the rock. On the other hand, it would make sense to shoot Blondie because he is the best shot (but did Angel Eyes know that?).