Runners on first and second, popup to shallow LF. LF easily gets to ball for catch, but ball pops out of mitt. Picks up ball, throws to 3rd in plenty of time to record force out. Of course, third baseman misses catch (good throw), with ball rolling to fence.
Had the third baseman recorded the force out, I would have ruled a FC (no error on play).
But in this case, should there be an error on the RF (batter reached due to drop) and 3B (runner from second safe on drop) or should there be only 1 error on 3B?