what is the version you running in RV320.
As long as VLAN 1 and VLAN 2 working, that means VLAN 3 also should work.
Make sure you VLAN3 device configured gateway as 10.1.8.X/24 (10.1.8.X - is the VLAN3 IP you configured in RV320).
what is the error you getting, when you try to reach from VLAN 3 to VLAN 1 and 2 and Vice Versa ?