A fixed number of winnings is reasonable if you want to make the chance to win appear reasonable to a wide number of members.
But assigning a seemingly low number of 3 max seems unreasonable from another angle.
With a fair amount of prizes limited in appeal based on geography and/or category, it takes some experience with the system to decide what prizes to pursue. You lose focus on the prizes and you can win and waste up to 3 chances on meaningless prizes. I've "won" gift certificates to more than 1 thing that were miles from me, and/or of little interest. Those experiences meant I could win a max of 2 in a month that would be of use, and connect me to the program.