So my understanding is that Boost will not activate any phone on their network that doesn't have an ESN (Electronic Serial Number I think it is) that's associated with Boost Mobile. Since your Sprint S3 has a Sprint ESN they probably won't activate it. People on that link are talking about spoofing ESN's and frankly I had no idea you could do that. I had thought the representative at the store when you come in to activate the phone is obligated to read the ESN that's printed on the sticker underneath the battery,
So the short answer is no, if you go to a Boost store they will probably not let you activate your phone using the ESN that's physically printed on the device (DEC/HEX) underneath the battery. To pull off successful activation you would have to A. find some ESN that Boost likes, B. alter the inner workings of your phone to convert its digital copy of the ESN to your new ideal ESN, and C. never physically show your phone to a Boost representative when trying to activate it. OR, you can D. get really really lucky and have Boost successfully activate your custom-ROM S3 on its original Sprint ESN. All this information is true to the best of my knowledge.