Most people find the third buckle unneccessary, as several have already said here, it's most likely a throwback to military kilts which had a much higher rise, and the two buckles were needed to make the kilt fit well. These days the lower buckle just isn't needed to hold the kilt against the body. Many people still enjoy wearing a kilt with 3 buckles, but on the few occasions a client has asked me to put that buckle on, within a few months, they come back and ask me to remove it.
On the inner buckle arrangement, several of us who make kilts chose that way of working,It's got nothing to do with the style of pleating, and I really don't see how it can be more difficult to put on, perhaps the first time, but after that it really can't be much of a problem surely.It's not Matt's creation, it does come from older ideas,infact some ladies skirts in the 1930's were fastened in the same way, so not that modern an idea either. I think Matt learnt it from Bob Martin.
I like the fact the buckle is hidden on the inside, it's a clean look, but in any case whatever style of left side buckle is used it would almost certainy be hidden by a belt