Post a close-up photo of the hole on the inside of the crankarm where the inner-rings would attach. Shimano doesn't list any replacement bolts for holding the inner chainrings on. They may just be riveted on and the rivets have pulled out on yours.

Easiest repair would be getting a new crankset. Can typically pick them up at shops for free or $5-15 bucks from their used-parts bins. College bike co-ops are also a good source for used parts. Or you can pick up an entire bike from thrift stores or garage sales for $10-20 and end up with lots of spares.

More difficult repair would be to tap the hole in the crankarm for bolts. Measure the bore-diameter and find the next size up metric threading. Probably 8x1mm or some such. If you don't want to drill all the way through crankarm, you'll need a blind tap.