iMac 27" Late 2014 Wont boot, no chime fan spins...

How many diagnostic LEDs are there?

The 3rd one indicates there is no communication between the GPU & Logic Board. You can try replacing the CMOS battery, but more than likely it will require replacement of the Logic Board or the GPU (if the GPU is available separately....more recent models Apple considers the GPU part of the Logic Board...I just don't know about the 2014 model).

Edit: If this iMac has a short flat cable connecting the Power Supply and Logic Board, then you can try replacing that cable just to be safe. I don't know whether the 2014 model has this cable. It will be visible as soon as the Display is removed.

You misunderstand....some later models have five diagnostic LEDs instead of just four which were used on older iMacs. I'm not certain which model started having five diagnostic LEDs. It may not make a whole lot of difference anyway.

My advice still applies regarding the problem and possible solution.

Apple doesn't care about supporting "Obsolete" products. It is so bad that Apple even removes documentation from their website once a product expires instead of archiving the documents for those of us who may still use their older products or who want to preserve them for history.

Besides any Apple tech you contact through official Apple Support links on this site will not have any clue about those Diagnostic LEDs since they are only meant to be used by the Authorized Service Providers and not the general public. Besides, a 2014 Mac is considered "Obsolete" by Apple so they won't help you anyway. The user community on these forums is always your best bet as long as someone with the necessary knowledge or experience happens to stumble upon your thread.

FYI, Apple also does not care about anyone's Right to Repair even though they "supported" the California Right to Repair legislation recently. It makes for good PR in the media though where the media doesn't really understand or care about the real problem. It makes it appear to legislators that the problem is now solved when it really isn't solved because of all the loop holes & tricks manufacturers can play with the rules. Sorry, got to be careful here.

I guess this is a logic board failure and that is very hard to fix yourself.