Stage 3 heart murmur

I'm a new guy so bear with me. I searched to see if I could find other posts on this, but while close, not quite there. My wife and I have purchased two MCs from a reputable breeder here in Virginia. They are both 5 months old now, male, and came with medical documents describing their parents. No problems. One of our kittens has checked out perfectly by the vet, however, our second kitten appears to have a stage 3 murmur. The breeder has offered to give us another kitten, and said we could keep the one with the murmur. She is also going to take the kitten to her cardiologist. Apparently, stage 3, if that is substantiated by the cardiologist, is bad from what I understand, with little hope for the future. The kitten tested out fine while with the breeder so this is something he must have grown into after his checkups. Does anyone have experience with this level of murmur. I am not going to throw my kitty away, and I just would like to know what to expect if possible. Alternatively, if their is a thread I missed that covers this, please direct me. Thank you.

The fact that the babe checked out ok with the breeders vet & she is prepared to get further investigations carried out I would sit on the fence for a while & wait for the kittens visit to cardioligist.... Does babe appear happy or do you think think it appears quieter than the other one...? Heart murmurs can be caused by many factors,some quite straight forward such as stress from change of homes plus trip to vet, infection...... my daughters has had grade 1 from when she had her progressing to grade 2 has been grade 3 now for a long while,cat now 16 yrs old & still not showing any signs of CHF so is still not on any medication,it does all depend on what is causing the murmur so everything crossed it isn't the HCM which obviously is what everyone is thinking / dreading. I am sure if it is the worse the breeder will be there for you & help guide you in any decisions you have to make to help your babe,just so pleased you are prepared to keep babe too,shows a true animal lover & everything crossed here for you all....xxx

As an add on to above although not from birth I have one MC coming up to 8 with grade 3 heart murmur & a 12yr with a grade 5,neither are on any medication as yet,two lines of thought do you medicate to help off set for longer signs of CHF or wait until it is required,along with daughter I have taken latter track & neither are on any thing & coping with life fine......x

Thank you both for your responses. My kitty seems to be just as healthy as his pal, both being 5 months old as I mentioned. They are like a herd of elephants running around our home and I cannot see any difference between them. My breeder takes him to the cardiologist on Wednesday, and has suggested we take everything one step at a time. I have complete trust in her. I just do not understand how I could have become so attached so quickly. Whatever the outcome, we are going to make how ever many days he has left the best he could possibly have. Thank you again for your responses. They have made me feel a whole lot better.

Well, I am back. Ceasare is still with us. As it turns out he has a small hole between the ventricles of his heart, but I don't think he knows it. Our breeder gave us a female kitty which now gives us 3 Main Coons. We have named her Anakatsia and she rules the house. Someone told me that female kitties are less affectionate than male kitties, but Anakatsia did not get the memo. Anyway, the reason I am back on here is because Ceasare seems to have another problem that I would like to know if any of you have encountered, and is it something of which to be worried? He has one blue eye, and one green eye. The iris in the blue eye is always larger, or more dilated than the green eye. If I knew how to attach a photo I would. Thank you for any response you may want to share.