i don't see the middle antenna being a PIFA based antenna. The middle antenna is a PCB etched style antenna, not PIFA, atleast it is when i take apart my own routers, and this is with either the 300 or the 600 series.
With PIFA - there would be some kind of antenna in the case that the middle antenna connector would be hooked to, if you look at internal board pics you'll not see such, only a PCB etching that ends of onto the board, no wire from the etching to connect to any kind of antenna. If it is infact PIFA, it would have to be from the board itself, which i wouldn't see a need to bother, an etched style antenna into a board will give hardly any gain, and even with the worst PIFAs you'll get atleast 1.5~2dBi gain - and this is with a physical PIFA antenna. With no physical PIFA - i wouldn't see no more than .2 dBi gain - so like i said, why bother..
The middle antenna is the least of your concerns though, as i said, it really doesn't work. But why it doesn't work is a bit more technical. It functions as a Rx antenna only, but your 2 end antennas have both Tx/Rx streams, thus fitting a 2x2 MIMO, the one in the middle could only be used if you had a 3x3 MIMO adapter because you couldn't get the 3rd stream with a 2x2 adapter, and here another issue, the router itself is only a 2x2 unit, so hardware limitation wise, it couldn't use the 3rd stream anyhow even if it wanted to - hardware wasn't designed for it. And the manufacturers probably know this, thus why the middle antenna is a PCB etched style with no use...
And for the record, its not just buffalo units that are arranged like this, i have an asus RT-N16 that is exactly the same, except those guys even went out on a limb and even tossed in an actual physical external antenna that does (guess what) absolutely nothing, and this was even tested within the dd-wrt forum and proven, changing the middle antenna on it did nothing, changing the left and right ones gave great gain in the overall signal.