There is so many holes in the unfortunate series of events I see little or no consistency other than it seems the OP has been so unlucky to get 3 faulty logic boards which leads to suspicions on some inherent design issue, yet not, as many of us have early 2015 MB that still function.
The OP states he bought from Amazon in 2015 but we have no idea if this was an authorised dealer or just some grey import
The OP failed to exercise his consumer rights correctly and circumnavigated his sale of goods contract and bypassed Amazon and possible CC company and went straight to Apple
The original Logic failure was outside warranty period
The OP agreed to replacements that only carry a 90 day warranty
The successive failures were also outside warranty periods
This is what I gleam from the OP postings
Even under UK or EU law reverting back to Amazon or Amazon seller that ship has sailed the OP never gave them the opportunity to repair and presumably they have nothing to do with successive logic boards
Under UK law after 6 months it is your responsibility to prove the fault this is why probably Apple will not give details on why the board failed as consumers could collect data and use it against Apple
This has more chance as a mercy case if you can get someone high enough up the Apple chain to listen
Else
You need to collect 100's if not 1000's even if anecdotal accounts of similar issues and use the small claims court against Apple . Even then your compensation is likely to be small given age wear and tear etc etc
Its a shame for the OP but many have suffered until class actions against Apple have been successful.
Even then as we have seen many are more convinced that Applecare is a necessity so Apple wins again despite the negativity of this posting and others
The OP states he bought from Amazon in 2015 but we have no idea if this was an authorised dealer or just some grey import
The OP failed to exercise his consumer rights correctly and circumnavigated his sale of goods contract and bypassed Amazon and possible CC company and went straight to Apple
The original Logic failure was outside warranty period
The OP agreed to replacements that only carry a 90 day warranty
The successive failures were also outside warranty periods
This is what I gleam from the OP postings
Even under UK or EU law reverting back to Amazon or Amazon seller that ship has sailed the OP never gave them the opportunity to repair and presumably they have nothing to do with successive logic boards
Under UK law after 6 months it is your responsibility to prove the fault this is why probably Apple will not give details on why the board failed as consumers could collect data and use it against Apple
This has more chance as a mercy case if you can get someone high enough up the Apple chain to listen
Else
You need to collect 100's if not 1000's even if anecdotal accounts of similar issues and use the small claims court against Apple . Even then your compensation is likely to be small given age wear and tear etc etc
Its a shame for the OP but many have suffered until class actions against Apple have been successful.
Even then as we have seen many are more convinced that Applecare is a necessity so Apple wins again despite the negativity of this posting and others