No, you don't.
Pastry flour is usually low in protein/gluten compared to A/P or bread flour. It may also be more finely ground. Both help in achieving a light, delicate texture when baking cakes, cookies, biscuits, etc.
But it's not necessary. Especially if they're asking for a 3/1 mix, you probably won't even notice the difference. Just be careful not to over-mix the dough: this encourages gluten formation and forces out air, thus resulting in tougher, denser cookies. Letting the dough rest in the fridge or freezer for 20 minutes prior to baking can help avoid this as well.
Alternately, substitute oat flour (grind some oatmeal in a blender / spice mill if you don't have any) - it has no gluten, and will add a pleasant, nutty flavor to the cookies.