If they released 3 sets at one time, you can expect less people to buy from each set because 1 person would buy just 1 set out of the three.
But if they release one set at a time, people would buy that 1 set, and then a couple of weeks later they would buy gcoins again, and buy the next set (assuming the sets are nice). This way they make more money.
(^does that make sense?
Just like when they release book series or something, they release one, let that one make goooood money sinec everyone is buying it.. and then release the next one so everyone would buy that one too...
Although that might have not been a good example.