From the picture, it doesn't look like the spoke has broken, but the threaded nut that goes thru the wheel hub and screws onto the end of the spoke has come off. This usually happens when a spoke gets loose, if you don't check them regularly once they are loose the nut just unscrews and comes off. I check mine for tightness every time I do an oil change, only takes 30 seconds. I doubt much harm will come from riding with 3 missing spokes (assuming they are not all missing from the same spot), but I would still avoid using the bike until I had replaced them. I don't know if you can buy just the special nuts from Yamaha, or if you have to buy the full set of spokes and nuts, as stated some people on here have sold spokes (again I don't know if they came with the nut), you need an allen key to tighten the nut, or a spoke spanner).