How to combine multiple tun interfaces?

Question

linux

Good afternoon! Help me please.
There is a server on Ubuntu Server 16.04.1.
It has 3 Open VPN servers, respectively 3 tun interfaces (tun0 (10.8.0.0), tun1 (10.8.1.0), tun2 (10.8.2.0)).
The next question is how to make sure that all clients have access to each other?

Answer 1

man route