Magic. Either someone cast a duplicating charm on her 3 times (or maybe she cast it on herself, after all who knows Emma better than Emma?), or it's 3 or 4 others using polyjuice potion (hmm, need some of Emma's sweet hair for that to work), or my personal bad fanfic answer is that 3 future Emmas are using time turners to return to the present.
I suspect either she cast a duplicating charm on herself 3 times, or 3 future Emmas are using time turners to return to the same time as present Emma. Or it could be 3 (or even 4) others using polyjuice potion.