I finally have 4 people in the family diving now. However, diving solo looking for a buddy has good and bad points.
First on asking to be a third. If it was me and one of my daughters, no problem. Me and my wife who is timid and nervous, I would see if there was anyone else you could buddy with.
Even with all of the divers now in the family, I tend to dive a lot more and need a buddy 50% of the time. I have never been without,, save once where the DM said to buddy with him. Usually not a good buddy, as his focus is not you but the entire dive group. You must pretty much stick with him, no chasing the fish.
That being said, of the buddies I have had, 90% of them have been interesting people to dive with. Talk lots with your new buddy on the trip out. How you dive, ensure signals are understood. Even get a translator if required. The best instant buddy I had didnt speak the same language as me and I had to use my wife to translate. We did 3 more dives together, and still keep in contact, my Italian is slowly improving although not as fast as his english. The 10% bad, well dont sweat it. Chalk it up to learning. The things I didnt like about some instant buddies is the type of same ocean buddy, so I simply resort to following them. Even then you can learn something maybe they search for different things swim too fast or too slow. Just dont buddy with them again.
Every time I do an instant buddy, I ask when they are going next. That way, if you liked them as a buddy, you might be able ensure you go at the same time, or if you didnt enjoy the dive, then that dive might be one to avoid.
The only real sad part about the poor buddy is that I tend to get focused on that experience, and it usually takes my wife to remind me to forget it and remember the great dives.