I'm assuming you're talking about the Dawn Stone on route 3? Then there are three other methods for obtaining a dawn stone in XY:
1. Battle Psychic Inver on Route 18 and hopefully he might give you an evolutionary stone, and hopefully it is a dawn stone, however this probably the hardest method.
2. Win a Dawn Stone in "Secret" Super Training. If you've never used super training, then you might have to play a few games to reach this mission. The mission is titled, "These Long Shots Are No Long Shot!" with a Gallade Balloon as the target. This method is probably easier than method 1, in my opinion.
3. Simply trading for it with someone else or with another game you have.
Hope this helps. Good luck finding one, finding extra stones can be frustrating.