These three churches are in serious trouble. Sardis is about to close down. Philadelphia has an open door, but little else going for it, and an hour of testing is on its way. Laodicea is lukewarm.
Both Sardis and Philadelphia are to strengthen what they have (Rev. 3:2, 11). No matter how weak, they are responsible for what is left.
Laodicea is, of course, a warning to every rich church. They didn't know what trouble they were in. They are to buy from Jesus -- gold, white clothes, and eye salve! Christ rebukes and trains those he loves. Therefore, they are to be zealous and repent.