Line array principal is dictated by the idea of a line source, with a 3dB drop per doubling of distance up to a certain point. This obviously goes against the inverse square law, but does so through the use of constructive and destructive interference. THIS is where the L1 does
work, it does control it's vertical dispersion of low-mid frequencies in the same way larger scale line arrays do. The problem is that the smaller the line-array, the less effective it becomes at lower frequencies, and this is where the L1 falls down. Such a short 'line-array' cannot, and does not have enough scale to affect the dispersion effectively at the frequencies we need it to. You would need to double the height of the L1 in order to add significant directivity on the vertical plane.
Secondly, think about where an L1 is going to be used. Stood on a stage next to a guitarist. Whoopdy doo, we've stopped a little bit of sound from hitting the ceiling (note, not much, only a bit) and bouncing off the floor. What about the sound which will wrap around the cabinet on the horizontal plane from really low frequencies to really quite high frequencies? The L1 offers NO control over this except through the use of the narrowing of it's drivers at higher frequencies (which results in a very, very ragged response even perfectly on-axis, with massively varying sound as you move up and down in front of one).
Ultimately, technically the L1 is a line array throughout a very narrow frequency band. Practically, the L1 shouldn't be considered a line array because it doesn't give you half of the advantages a properly designed line-array would. At the same time, it doesn't give you ANY of the advantages of a decent point-source system.
Lastly, 3" speaker drivers for PA? You must be having a laugh. Even if you take into account driver coupling, you'll never get a decent level out of that.
EDIT: I really haven't got the time to put references in, but here's a link
which basically explains line-array theory in it's entirety (more or less).