General Questions

Here is my config :

                                                      -- ip B (vlan 2) ----- PC 2 (with ip B for gateway)
PC1 --- IP A - monowall --- trunk <
                                                      -- ip C (vlan 3) ----- PC 3 (with ip C for gateway)

When i ping PC 3 from PC 1, i get a normal answer, icmp echo reply from PC 3.
When i ping PC 2 from PC 1, i get an answer from the IP A ...

Of course, the 2 vlan are configure exactly the same way.

Bug or wrong config ?

That's expected behavior, you need to disable NAT (see FAQ) if you want to strictly route, which is what it sounds like you want.

Thank you, it was the solution indeed.