In the simplest sense, for V3 to make 1 he either had to have jumped rank, or V2 would have to have been removed. Both seem plausible to me.
At the point of V1's demotion and arrest, V3 was heading the most important yeerk operation. The council basically said no one else was available to do it. It would make some sense for the yeerk leading the main effort of the empire to fill the highest rank when it became open. In that case V2 probably would have had resentment to V1 like Snakie suggests, but he could just be playing politics.
Or
Visser 2 was killed or also demoted at about the same time V1 was. It makes sense since it seems like the yeerks lost ground very quickly in the war after the Anati defeat. There'd be many ways they could have lost both. For instance, V2 may have been a fleet commander who was tasked with supporting the Anati defense. V2 would have either been KIA or summarily executed for the defeat like V1. In which case the vissers would just cover down.
I'd think the second is more likely since, but it wouldn't bother me either way.