He's only three so it's absolutely not too late, but it's just something you need to work on via training - there's no magic solution unfortunately.
I would look at "capturing the calm" - basically, if you see your dog behaving calmly with no prompt from you, reward them, either with a treat (nothing exciting, just place it near them) or with calm, gentle praise. The idea is that they then do more of that behaviour as they learn it brings good things, but they learn to do it naturally rather than by force or pressure from you.
Adding onto that, you want to teach a "settle" command. Start at home and encourage your dog to settle on a bed or blanket. Once they do so on command, you can move the command to more distracting environments - the garden, the park etc.
Then, ideally, when you go away, your dog will want to behave calmly because they know it's what you want from them, iyswim.
However, saying all that, my dog would be bored stiff just wandering around a museum - I mean, practically speaking, what's in it for the dog? So beforehand, I'd be making sure he was tired, fed, toileted and well-exercised so he wasn't "needing" anything :)