Rule Information tab:
a) Rule Name
b) select Status
c) Select Websites
d) Customer Groups
e) Select Coupon: Specific Coupon
f) Add your coupon code or auto generated coupon code
g) 'Uses per Coupon' field as per your requirement
h) 'Uses per Customer' field as per your requirement
i) 'From date' & 'to Date': you can leave blank if you want always apply this coupon on the cart.
If ALL of these conditions are TRUE :
a) apply: as per your requirement
b) Discount amount: as per your requirement
c) If ALL of these conditions are TRUE:
Category is 3 // category id may be different.
Hope, it will be helpful.