Hi all I've setup a Lan To Lan between two offices & now want to put a third into the equation, any hints or diagrams how this is done?

I'm no expert, but in the past when I did this I just created IPSEC tunnels between all three sites, so A <-> B, A <-> C, and B <-> C. Each site had it's own subnet e.g. Site A 192.168.10.0/24, Site B 192.168.11.0/24 and Site C 192.168.12.0/24. This was using a different brand of device (Cyberguard), but they were just IPSEC tunnels, so I assume it will work with the Draytek devices as well.