You are exactly right. This is the tricky part because the player or audience does not know whether the host knows where is what and thus observing him revealing any envelope does not affect the probability of the outcome.
If the host already know where is what, then he knows the player made...
Seems like the Monty fall problem is similar to reducing the Monty hall paradox to a two envelopes puzzle (watch YouTube)...yes, in this case the host is just like a second player who does not know where the top prize is, but this second player only have 2 choices to make whereas the first...
There are three envelopes. One contains $50. Another contains $100. The third contains $500. You pick one but it is not opened. If the host reveals one of the other two and it contains $50 or $100 (both considered as goats), the answer is to switch as in Monty hall case. However, if he reveals...