manually port forwarding for my router

[AdamN]

Different games uses different ports, which will also be different depends on the Port Offset value, so manually port-forwarding can be tricky for most people (unless they already familiar with the ports used by the game).

If even DMZ doesn't works for you, that mean your ISP is blocking the port, thus manually port-forwarding nor UPnP won't work either.
The only solution for ports being blocked by ISP is by using VPN.

However, since you're able to play with one other player successfully, that means both of you doesn't have blocked-port issue.
There might be some other issue, probably the 3rd player had issue, unless if that 3rd player were able to play the same game successfully too with another player.

For example:
1) P1 <--> P2 = worked ==> no technical issue
2) P1 <--> P2 <--> P3 = broken ==> issue on P3
3) P2 <--> P3 = worked ==> the route between P1 to/from P3 might be bad, where the route are depends on the ISP if you're not using VPN, and depends on the VPN & ISP if you're using one.

PS: There is also an issue where multiplayer with more than 2 players doesn't work well when using a certain VPN on some games, but since you're talking about port-forwarding, i assume you're not using any VPN (since VPN doesn't need port-forwarding)