How to implement routing of transit ip packets through Ubuntu 16?

Question

ubuntu

There are three Ubuntu machines, one of which is intermediate, the two outer ones need to be connected in VLAN 90 to the network 172.16.1.0/24. The intermediate machine has two interfaces 192.168.1.2 and 192.168.2.2, the outer machines have 1.3 and 2.3, respectively, and the outer machines also have a VLAN interface. The default route of the end vehicles is the intermediate vehicle. All addresses are pinged from the intermediate machine, and both VLANs and 1.3 and 2.3. But VLAN interfaces do not see each other through an intermediate one. ip forwarding is enabled. Why do I better not ask, you need

Answer 1

Can you see /etc/network/interfaces?

Answer 2

Do not see each other IP addresses 192.168.1.3 and 192.168.2.3?
Or do you want them to see each other at addresses from the 172.16.1.0/24 network?