Re: Christmas $$ issue
I can see why you are feeling a little bad about this, and if they were all 7 yr old triplets that knew what things cost I think it would honestly be a different story (at least, it would be for me). Some people might still feel the same. But if the kids were old enough to understand the significant cost/price difference, I could see there being some hurt feelings from the other two that one of the children got a much much more expensive gift.
So, that said, since your younger two are only 2 and 4, I do personally think it's ok to spend more on your older son to get him the gift he really wants. The other two are getting their "want" met as well, so the cost isn't relevant in this issue given the age difference.