If you have only had them for 3 months it really isn't enough time for them to have settled in for them to be comfortable to breed, it can take up to a year for a breeding pair to be at ease to breed in a new environment. It must also be the correct time of year to breed, greencheeks are seasonal breeders and will only breed in the spring/summer season.
Birds also breed best when they live outdoors, depending on where you live in the world this may not be possible due to climate but the natural light, and weather play a huge part in breeding success.
Diet is another contributing factor, ensure there is plenty of nutrient rich foods, leafy greens, sprouts, almonds, a calcium supply (ie cuttlefish or calcium supplement) it is also a good idea to switch from a maintenance pellet to a breeder or high energy pellet a couple of months prior to expecting your birds to breed and throughout the breeding cycle.