Switch from 3×9 to 1×9

Unless the current drive train is worn out, I’d just keep the 3x until you are ready to buy a full 1×11 or even a 1×12 group (grey market complete groups on Ebay are not that expensive). To get the best benefit of 1x you want the narrow-wide chainring, a clutched RD, the wide range cassette, a shorter cage RD, etc. If you just try to work with what you have then you sort of get the worst of both worlds without any advantages. Save up first, do it right from the start.

This is one of those weekly questions, so I’d suggest searching the sub for “conversion” or “1x” and see what pops up.

Current crankset has bolt on chainrings.. maybe 104mm BCD for the big and middle rings. You can just put a narrow-wide chainring in the middle position then remove the other two rings (or put a bash guard on the outer position) and remove the front derailleur. If you don’t use a narrow-wide chainring, I’d keep the FD as a guide. WolfTooth has a chainring guide you can print out to determine the bolt pattern. Or google “Sheldon Brown how to determine chainring BCD” for a guide on measuring it.

If/when you want to get a new crankset, then you get the necessary bottom bracket for that crankset at that time.