Networking between 10.6.8 (Snow Leopard) and 10.12 (Sierra) For work I have to solve an issue involving very old and new systems. Because of legacy software required for the business, I need to sync folders between Mac A (MBP running 10.6.8) and Mac's B and C, both are iMac's running 10.12 Sierra. 

I had been using AeroFS before they shut it down and it worked great. Simple folder mirroring. *I want to mirror folders so there are fewer points of failure, and all files are on each system. 

Bittorent Sync (Resilio Sync) would work PERFECT except 10.6.8 is too told to run the client.

What is the best way to mirror folders between these systems? I've considered Dropbox, but due to file sizes and slow internet has been ruled out. 

Would a NAS be ideal in this situation? How can I best mirror folders between all 3? 

EDIT - Mirroring folders is needed as the MacBook Pro is mobile and will need file access away from the internet.
ChronoSync should be able to do what you want.  While they offer an older version that will run on 10.6.8, I would suggest that, instead, you run the latest version on your 10.12 system and use it to sync the files amongst all your systems.