Since this is a question of preference, I prefer two straps/buckles. I find the third has a tendancy to pull the apron in an unnatural way. But I do have kilts with 3 and I don't much fuss about it.

I will echo the left hip INSIDE buckle comment. I just got a nice kilt with this feature. Asthetics aside, it is much easier to buckle while getting dressed--that is probably just me, but this was what I noticed immediately. I can reach it more easily. Just an extra thought or point that I have noticed about myself.

I will say that I do not believe the number of buckles is paramount to formality. Nor am I a believer that the third strap/buckles is the current evolution of the tradition.

What I do believe is that kilt makers do as their teachers did. Or as they were taught. End of story. I believe a good number of kilt makers here do two buckles as their standard whereas others do three. It is what it is.