Converting 3x7 to a 1x7. Is it possible with only existing parts?
You have a zero cost solution, which is to keep the front derailleur but remove the cable (and shifter if you like). Then use the FD limit screws to fix it in the right position to act as a chain guide.

You don't need a 1x chainring, in fact using one with a 7 speed chain is probably less than ideal, so you're actually better off sticking with an existing chainring. You may not be able to get rid of the big chainring, or you may need to get shorter chainring bolts if you do, depending on how the big and middle rings are mounted.