For either DPPt or X&Y.
And soft-restting for a 3DS is basically clicking the Home button and closing the game w/o turning the 3DS off.
(Apologies if this has already been asked)
Yes,it wil.I don't have any particular source but I have tried it and it does break the shiny chain.
Hope this helps!
( And I only tried it in X and Y,so I don't know about DPPT but I'm pretty sure its the same)