Indeed, there was even a handful of slot loading macs that could handle 3" CD's. But most can't..
I wouldn't try one in a slot drive with an adaptor ring either.
I also have a few 3" cd's.. Fortunately I have quite an old Firewire cd burner drive that can take them, as its tray loading. So I can always hook that up for the rare occasion it's needed.
try an external usb disk drive like the macbook air has but the one that you can get for the air wont work for your imac so just try a mac compatible external disk drive
Your best bet is not to spend $50 on a drive for a couple of disks, rather to copy the data off, rip them, or whatever, on another computer, say - handy PC - then USB stick/full-size CD them over.