@jucat60
Unless they do tissue or blood testing they cannot say categorically that it is Marek's and to me, there is no point in paying to get a necropsy done if you don't get a definitive result from it. There is usually the option to authorise further testing within a set period, usually 28 days, so I would suggest you enquire with your vet about the cost of that testing and if you can afford it, then get it done. It is better to know what you are dealing with and at the moment you still cannot be sure. Or find out where the necropsy was done and get in touch with them direct to discuss it.
The other 3 chicks might be resistant to Marek's, or they might have been infected but not symptomatic yet, as it can lie dormant for weeks, months or years, or it may not be Marek's, but if your only doubt is because the other chicks are still healthy, then you may be kidding yourself, I'm afraid. I've had Marek's in my flock for 4 years and I've just had a 5 year old bird come down with it earlier this year for the first time..... I have a mild strain where the first outbreak doesn't necessarily kill them. She has fully recovered after months of wobbling.