Do you have to reveal your hand when using Treasure Map and not trashing a TM from your hand?
A very similar question was posed on DominionStrategy.com (although now of course I can't find the link to the specific discussion). The thread was about paradoxes in which you aren't required by the rules to reveal your hand, but then your opponent couldn't verify that you in fact followed the directions on the card. The Treasure Map scenario was one of the examples mentioned in that thread; another example was Moneylender (which requires you to trash a Copper from your hand but has not clause about revealing a hand with no Copper).

Having said that, let me first walk through what must happen in the scenario you outline, and what Donald X. (the game creator) has said about the potential for cheating in that scenario.

Your scenario is having a hand of Throne Room, three Treasure Maps, and a Copper. If you play the Throne Room, you are required to choose an Action card from your hand. The only choice is a Treasure Map, so you play that card twice. Both times, the card directions tell you to trash another Treasure Map from your hand, so you must do so both times because you have Treasure Maps in hand. The if clause is only satisfied the first time, though, so the second play of the original Treasure Map (from the Throne Room) does not cause you to gain the Golds a second time.

As to the potential of cheating, Donald X.'s response (in paraphrase) is that it would be ridiculous to specify absolutely everything about the game, such as what constitutes valid shuffling (yes, this has been an intense discussion on DS.com), what the penalty is for breaking the rules, etc. In short: if you cheat, you're already breaking the rules; what would be the benefit of having a rule for what to do if someone breaks the rules?

The way the Isotropic server implements these situations is that the server requires you to take the actions that a card demands, and so it provides an objective enforcement of the rules. Because Treasure Map doesn't say to reveal your hand if you don't have a second Treasure Map, Isotropic doesn't show your opponent your hand. What your opponent would see is something like:

X plays a Throne Room
…and plays a Treasure Map
……trashing the Treasure Map and a Treasure Map from his hand
……gaining 4 Golds on the top of the deck
…and plays the Treasure Map again
…trashing a Treasure Map from his hand

If you're playing in an environment in which player's honor is not enough of a guarantee to avoid cheating, then that environment could specify additional procedures (such as having an ombudsperson to verify that the hand was played legally or using Isotropic, which would enforce the rules). You could specify a house rule of "reveal the hand if you can't follow part of the directions," although that could potentially give the opponent additional information that they wouldn't have otherwise. But, that's all up to you. What the rules require is that, if you play the Throne Room, all three Treasure Maps will be trashed and if you don't do that then you're cheating.

Do note that, unlike what some posters below have said, the Treasure Map card should not have been written with a "may." Donald X. deliberately made the effect non-voluntary. He is very careful about getting card directions right, and that's one area where there's no ambiguity. Claims about the "templates" for other cards in the set miss the point; when Donald wants an effect to be voluntary, he uses, "You may." Because he has a clear template he uses for other cards, you can rest assured that, since he didn't put that into Treasure Map, he didn't want that direction to be optional.