Hello - when is your tour scheduled for (i.e. what month)? And do you know if any of the days you would be visiting these places will be weekend days?
I say that because I think it would be most important to purchase ahead for
Chateau de Chenonceau
Chateau Chambord
particularly if you are going on a weekend day anytime from say April through September. Just to be sure that you wouldn’t get there and not be able to get in, that would be horrible !! (And I don’t think you would have to buy these tickets WAY ahead of time — surely a week or two ahead would be fine. I guess the best thing to do would be to look at ticket availability on their websites for tomorrow, and for two weeks from now — for example — to see what ticket inventory looks like. That can give you an idea although of course any given day may be different.)
I don’t think it would be necessary to purchase ahead for Clos Lucé, and not for Chateau d’Amboise.
I don’t know about Pont du Gard, it’s been a loooong time since I have been there. Hopefully another member will be by soon who has more recent experience !
I don’t know anything about Monaco.