Is this current source (or mirror)? How does it work?
R19-C20-D3-R18-Q2 form a current source.

The rest of the circuit is easier to understand by moving transistors around. The original schematic looks weird, presumably because the transistors are drawn as they are connected inside LM3406. It is simpler this way:



This is a Wilson current mirror with a ratio of 2:1 as it has two transistors in parallel on the output side. 

The first current source around Q2 should output around 1.9mA, thus the mirror should output double that, around 3.8mA which then feeds differential pair U2B/U2C.

Now, why they chose this design is a more interesting question.

Apparently LM3406 is a LED driver and not a transistor array, so there must be an error in the part number... and I can't find a datasheet.

My guess would be that the Wilson mirror has a much higher output impedance than a single transistor current source, so its output current stays much more constant when collector voltage varies.