They were switching one striker for another, though, weren't they? I think putting on an additional (third) striker makes very little sense unless you're basically hoping to give yourself more chance of being first to any loose balls in the goalmouth. What tends to happen is that one of the three ends up playing in a withdrawn or wide role anyway, in which case you'd be better off bringing on a player more suited for that role - assuming you have one. Bringing on a second striker when you're playing 1-upfront makes a lot more sense, though.