I've managed that before more often than I'd like to admit.
Normally you can wiggle the chain free, though it does take some force and may damage paint. Pull some slack in the chain by moving the derailleur forwards, or it will pull back on the chain you are trying to free up.
You've tried removing the middle and outer chainrings, but I think not the little chainring which is the one pinching the chain against the frame. This has four bolts of its own holding it on, accessible on the inboard side of the small ring. You can remove these one at a time while the crank is still fitted. It should give enough play to release the chain. I would try this first but if you can't rotate the crank to access enough of the bolts then it might not work.
So that gives option 2, removing the crank. To remove the whole crank needs a tool called a cotterless crank puller, cranks won't come off on their own just by taking the bolt out. Very searchable online for instructions. With the right tool available you'd probably just do this first anyway.
Remember to do all the bolts up tightly again. It's a good opportunity to clean them and add a little grease to all the threads.
If the chain likes to fall off the inner chain ring it needs looking at. maybe a tooth is bent, or the chainring is worn, or the limit screw on the derailleur is not set properly.