3 Mbps is certainly not enough to stream and game at the same time. If it's working fine on stock firmware that's because you have it set up to prioritize for the stream and are lagging in the game. You are not going to be able in most cases to observe your own lag while playing against other players. However, they will. So your streaming may be fine, but other players will observe your lag. Most connections to be considered a "good" connection for gaming are at least 4-5 Mbps upstream. That's just the connection for the game. Streaming uses a lot of bandwidth on top of that:
Upload Speed for 1080p: 3.00 - 3.50 Mbps
Upload Speed for 720p: 1.80 - 2.50 Mbps
Upload Speed for 480p: 0.90 - 1.20 Mbps
Upload Speed for 360p: 0.60 - 0.80 Mbps
Upload Speed for 240p: 0.50 Mbps
You should not stream unless you upgrade your upload speed at the bare minimum. Even though you may not notice it in game, it is most likely costing other players lag issues when playing against you. That is unfair to them. Even streaming at 480p there would most likely be lag with a prioritized stream.
If you are dead set on streaming without upgrading your connection in some cases using alternate DNS servers may help. Public DNS servers like Google and OpenDNS are commonly used by many gamers. However, your upload speed seems to be the underlying issue. I have a WR1043ND V3 using LEDE 17.01.2 running 20 Mbps upstream and have zero problems when I stream in 720p. It's perfect quality 99% of the time. My connection to the game, quality wise, is never an issue while doing so either.