So I have this crank that I want to convert. It has three crankwheels and I don't expect myself to ride on the two of them. I have found a second hand crankwheel from a friend that I want to put on so that my life gets easier but I need the "plates" or I don't know how they are called to come off. As I can see there is a bolt for a hex but when I put it in to turn it and remove it, it does nothing. From the back side of it, it seems to be a rivet. Ps. I don't have enough money for a new crankset if this is what you are suggesting. I just need the crankwheels to come off.
You should be able to remove the granny gear (the little one) and try running on a double for a while.
If the chainrings are rivetted together then you'll need a drill, and reverting will be impossible.
If they're bolted on with chainring bolts you need a special spanner, as per How can one remove this chainring?
If you remove the bigger chainring, there will only be the middle ring left, so no need to buy the replacement chainring.
There's a chance you can remove the front derailleur, but it might still be needed as a chain guide to stop the chain jumping off. Don't trash it when removing.