Should the log also show all discarded cards of my opponent then? I thought that it was fine to discard down to 3 and only show one card (new top of discard pile) to my opponent, if I don't want to trigger reactions.
From a post in a different thread (the bolding is mine (Mine is in Prosperity)):
I'd rather not have a special ruling and with no special ruling and nothing in the rulebooks, "discard down to 3" sounds to me like I keep discarding until I have 3 (or fewer dammit). However you don't need to reveal the cards below the top one because somehow the rulebook says this.
I've taken that ruling to mean you actually get to discard the cards in batches of 1–X where X is the maximum you could (must) still discard. So you can dump them all at once and reveal only the top one if there's nothing tricky going on. But you could also do it in batches of 1 and show them everything if you wanted to. Or something.
(Or else you're allowed to hide your discard pile while you discard things one by one, but I think it was ruled elsewhere that letting you hide your piles from your opponent would be a bad idea.)
When applied to Monastery, the grouping doesn't matter for information purposes (everyone gets to see all the cards that end up in the trash no matter how you do it), but Catacombs and other problem cards must be the last in a batch. So, I think, once you've included a Catacombs in the cards you've selected to trash with Monastery, instead of letting you add more cards to the selection, it should somehow say, "Hey, before you select more cards, let's go ahead and submit this batch now so we can handle tricky stuff. It's ok; you'll get to come back and pick the rest in a sec." Of course, it should still let you
remove cards from the selection (EDIT: actually, I guess it doesn't let you remove things now, but just lets you undo the entire selection). And if you remove the Catacombs, it should stop telling you you can't select more right now. And once you do submit a batch with a Catacombs, the Catacombs should always be implicitly processed last because clearly the player didn't select any new cards after that one.
The thing I don't like about Donald's suggestion of "select a bunch and then if something triggers, throw out all the other choices" is that someone who doesn't care too much about all of this is going to try to trash a Catacombs and some Coppers and then they'll be like, "Man, this game is broken; I told it to trash these Coppers and here they still are." It's best not to even let the player think they can tell the game to do something if that thing is not going to end up working.