Dragons are overpowered at TH7, so three starring TH7s is a relatively easy job. The only defences that do a lot of damage to dragons at this level are air defences. The key to a successful dragon attack is eliminating the air defences quickly. The most reliable method is to use three lightning spells to take out one of the air defences (ideally the hardest to reach, or the highest level one).
Attack the remaining air defence with your dragons. It is important to control the path of your dragons (funnelling). Dragons will target the nearest building. This can lead to them going in a circle around the perimeter of the base rather than in the direction you want them to go.
Use a few dragons to cut holes in the perimeter of the base so the remaining dragons have no other option but to path into the air defence.
Try to get some practice in on low TH8s when you can. These are more challenging, but still very possible with L2 dragons.