Thanks Gerald!
This was just as the wolves showed up, so I don't think the second horse had been killed already and if it had, Cuthbert wouldn't have known yet since he didn't know there were wolves... but I suppose that could be the case.
No, the second horse hadn't been killed already, there WERE still two.
The first horse breaks its leg on the bridge and is mercy-killed. The second horse is attacked by a wolf and it looks like its mercy-killed too - but the picture is not very clear, you can't see if Bert shoots the horse or the wolf. But whichever is the case, the second horse dies then, because Bert says (as if talking to the people at home): 'Oh, about the horses...'.
The third horse is still alive when they return home.
The confusion is understandable, it's not handled all that clearly. But I'm used to that in comics. Maybe it's because comics are often done by multiple artists, but I find mistakes/unclear things in them from time to time. More than in novels, although they can have them too obviously.