If we can convince one of those three to be our Manu, there is no reason to trade one of them. That is, if Harper becomes what everyone expects.
If none of them takes pride in coming of the bench or our cap situation forces us to, we still will have to wait for the trade to keep Fox and Klutch happy. Thus it also would have to be a soft landing spot and we don't know, what that is in '28. At this point it could very well be a rebuilding Clippers or Warriors or a restructured Heat team. It doesn't matter now, as we don't know if he will be traded, where he'd like to go and what that team could offer.
For now Fox is a Spur and we should be happy about it as we now have the tallest and fastest NBA player and both of them are really talented.
Would the Spurs do the Fox trade again, if they knew, they'd be able (nearly forced) to draft Harper? I think, they probably would, as the price was that good (it depends, what will happen to the Minny pick). Don't pretend, that Fox isn't a great player and could very well be our second option on a championship team.
Markkanen might be the better fit now, but I'm pretty sure Ainge wanted a lot more stuff.
After all it's less about assets itself but more of how we can make the salaries work. Maybe a change like cap relief for drafted players might help by then. I'd hope so, even though it would help OKC and probably Houston as well.