And how can you know what is most virtuous in every situation

How can you know the exact way to balance conflicting moral rules in every situation

You're conflating my claim that consequentialism requires an ideal observer with a claim that all other ethical theories are immune to any epistemological problems at all. I only said that they do not have that specific problem of perfect knowledge, implying the physical world. They do not need perfect knowledge in order to suggest action, but they would need metaphysics which is a separate issue.

consequentialism is kind of a subset of the other two.

It is a distinct normative theory.

Virtue ethicists mostly believe that aiming for good consequences (among other things) is virtuous

I would argue that it's more the "other things" category than good consequences, because a virtuous act is by definition good in and of itself, not good because of its potential outcomes.

deontologists mostly believe that aiming for good consequences is one of many moral duties

I have never seen this before, and it certainly would not make sense within Kantian deontology / categorical imperative. Do you have a source for this?

A consequentialist is just a deontologist who happens to believe the only moral rule is the duty of beneficence.

Then it isn't deontology anymore, because any act would be permissible as long as it yields sufficiently favorable outcomes.

If consequentialism fails in some way... then all major moral philosophies "fail" in the same way.

All major ethical theories do fail in some way, but not the same way as consequentialism like you said. Both virtue ethics and deontology can suggest action in the drowning-Hitler scenario without requiring knowledge of the consequences like teleology would. They might face issue if the agent had a conflicting duty, but that problem would not arise from lack of perfect knowledge.