IMOP, the older Q2's seemed a lot more user friendly and work much better then the G4's and since they came out with the Q2 Pro version that now adds in the bluetooth A2DP streaming, you are set. Of course, they don't do the simultaneous 3 or 4 way conversations, but in my experience, that is a crap shoot anyways.
My Dad has the G4 and my wife and I have the Q2's and it was always frustrating trying to get all 3 active at once. Once we did, we tried not to disconnect them so we didn't have to go through that again.
For the 3 way pairing, the person above pretty much gave you what you need. The one guy that has both riders connected (In your case rider 1), can open the connections independently by pressing the respective A & B buttons on his helmet. So if he wants both open at the same time, he should press one button, let it open, then press the other and it "should" open. However, I can tell you from personal experience, this does not always work. If you can't connect someone, best thing is to just turn the main unit back on and back off, let both riders "sync" up, then try it again.
If the main rider uses the VOX to open their connection, it should attempt to open both A & B connections at the same time if riders are connected to both. Again though, our experience with this multi-connection was less then favorable. It was great once it was open, but getting to that point reliably was a pain. I think some our issues was the mixing of the Q2's & G4's even though Cardo says its backwards compatible, I think they still struggle. It sounds though like 3 G4's doesn't help it much either though?
I have a buddy that just upgraded from the Scala Q2's to the Sena's and I don't think they are totally happy with those either and are struggling with them. I haven't personally used the Sena's and have followed them closely, but some people seem to love 'em and others are not so happy. After 2 years of use, my wife and I are still very happy with our Q2's. I think when they die, we'll just upgrade to the pro models to get the A2DP. I'm not sure I want to mess with a G4's after using my Dad's some. The VOX seems way more "finicky" on the G4's and takes quite a bit longer to actually open the connection compared to the Q2's.
One thing we have learned is if you were used to something like Autocomm's VOX or maybe something else where you just started talking, that doesn't work very well with any of these BT headsets. Cardo, Sena, you name it. They all lack that "immediate" just start talking VOX option. It appears most of the headsets rely on a combination hearing a substantial short but aggressive sound along with air directly hitting the microphone at the same time. So, what it takes is you coming up with a sound you can make that also generates a bit of air coming out of your mouth. We use a "beep" sound that we make by puckering our lips and making a quick blast of air as you literally make a "beep" sound. Once you hear the connection open, then start talking. Nothing is lost and it works pretty reliably.