Children don't have to do the same activities to be fair. Sometimes one will get oportunities the others (or another) didn't have simply because of something outside your control. In those cases, to me, it's unfair to say that one can't do it because the others didn't have that oportunites. Because in the long run, these sorts of things even out in the end.
The issue here is that it is in your control. You are saying that your second two can do NO activities because of the money (and I suspect time) that are thrown at your ds1. That is where resentment will come.
At present your two younger ones are you enough for it not to matter. But in the next year, you need to be thinking about how your ds1 can cut back in order for you to be able to afford something ds2 wants to do. Equally well when your dc3 comes up to school age, he will need to cut back again.
I know from my family growing up. All our family did a particular sport. Except my dsis who chose not to do it because she didn't enjoy it. Even so, so resented the time and money spent on it because, she felt that she came second best, if she had wanted to do a sport, she wouldn't have had the same time and money put into it, as there wasn't an unlimited pot.
Otoh I was picked from a group to have music lessons. Much as she would have loved to have been picked when it was her turn, she wasn't, and she didn't resent this because it was not something my parents chose to refuse to her.
For what it's worth, we refused a 50% scholarship for dd1's secondary, simply because even if dd2 (and later ds) had got the same scholarship, we couldn't have afforded to send them too. So it seemed unfair to give dd1 something we knew that the others couldn't do. We expained this to dd1, and she understood, and was happy with the decision.