You have been taking it to be that this trip of yours is reasonable in the context of a relationship but frankly it is not.

what?

a trip that was planned many years in advance, with ample warning, with a very specific time limit (3 months) is a no-go in a relationship?

it's not like they have kids and he's shirking his responsibilities. besides, we don't know his career choice - you're assuming that his career growth is hindered. my friend worked as a designer, and guess what - him going out there on a trip was actually a big talking point in interviews when he came back, he was seen as an adventurous person who was willing to do his own thing. his previous company wanted to hire him back as well.

Even if you told her day 1 of the relationship it doesn't count until it gets serious. Saying "I want to do x" Is not the same as "I have set the date to do x".

you're assuming that in 3 years, OP didn't say a single word about this trip. why? he says specifically in his post - "I knew she was not coming with me, but we always planned that she could meet up in different countries when she was flying there."

they have obviously had some conversations about this, if he's under the impression that she maybe able to meet up with him in different places.

But its not fair for you to just tell your girlfriend to deal with it or break up.

he's not even saying "deal with it or break up". he's saying - "this is a trip you know I've wanted to take for a long time. please join me at certain points in this trip, as I do my thing for a few months." he doesn't want to break up, but he is making it clear that a trip he has saved for ... for years ... and planned for, for a loooong time ... is one that he will be going on.

if you can't stand your ground in a relationship about something you've planned for years, with absolutely no mitigating circumstances (i.e. suddenly he says he's going with a girl, or he proposes to her but plans to use all his savings for the trip but not the wedding, or he recently cheated on her, etc etc) ... then you are in a very weird relationship.

the rest of your advice is fine, but the way you make it sound, he's somehow been a poor communicator. where are you getting that, besides making assumptions..?