I solved my problem.
First I created a new VLAN named "test_vlan" and bound it to port2.
When I opened the drop down menu of port2, there was shown the tunnel interface and the "test_vlan".
Than I changed the status of port3 to "up".
I created a software switch named "software_switch" and added port3 and "test_vlan".
After that, port3 and the VLAN interface of port2 disappeared and were shown under the interface "software_switch"
I made the same configuration on the second Fortigate 300C.
On both sides on port3, I connected a switch, which have a Port untagged to the same VLAN, also named "test_vlan"
and also having the same VLAN id. I configured another Port to the same VLAN on the destination switch, for testing the connection and - it worked!