I’m sorry to hear that you’re feeling upset by these things.
I’m in a friendship group of three. We all live c.1 hour away (in a sort of triangle) so when we meet up in tends to be planned in advance.
In earlier years, we would always meet up as a three, and we’d rearrange if any one of us become unable to do that date. It worked fine.
More recently, I think because we’ve all got slightly different circumstances and our lives have evolved, it is not unusual for any of us to meet one of the other group. We don’t hide it from the third person (who may not have been invited) and there appears to be absolutely no resentment from anyone. I think it works because we will sometimes meet up as a three and sometimes in pairs but it all balances out in the end.
So I think your resentment sounds unhealthy, jealous, immature.
If you didn’t like the sound of joining their holiday one day via WhatsApp, it’s absolutely fine to politely decline. By expressing that it would remind you of being left out, you’re likely to push them further away.
If I were you, I’d join the meet up four hours away and, next time, take the time to find a more mutually convenient spot and you do the organising.
I wonder whether these friends form the majority of your social life, and that might be why it affects you more to be ‘left out’ of an arrangement? If so, try to build other friendships, hobbies etc., while also maintaining this friendship.