The +3 doesn't know about High Density (1.44MB) diskettes so you need to use Double Density (720K) ones
, aka "blue floppies". These can be bought online, I don't know if they're still being manufactured anywhere or just new old stock. You can also cheat and tape over the hole on a HD diskette to make it appear as a DD one. This may or may not work, and the data may not stay forever (or it might be fine! but better safe than sorry if you save anything vaguely important on them). If you're just going to use a single diskette to "ferry" files from a PC to 3" disks then this is a perfectly adequate solution.