How to connect 2 different interface?

 Example:
Interface 1 = Lan 1
Interface 2 = Lan 2
Interface 3 = Lan 3

 

Lan 1 = 192.168.1.0 (Department 1)

Lan 2 = 192.168.2.0 (Department 2)

Lan 3 = 192.168.3.0 (Servers)

 

How can Lan 1 and Lan 2 connect Lan 3?