HELP! I don't treat my kids equally?

I have a 7y/o daughter, 12yo son and a 15 y/o son. I dont know why but I let my 15 and 7yo get away with things that I wouldnt let my 12yo do. I make him clean his siblings rooms as a chore and I expect strait A's from him while my other kids are allowed to bring home C's.I also punish him more harshly then I do my other children. However I do love them all equally and I feel like this has gone on long enough. Do you do this to your kids? And how can I change?? I need all the advice I can get.