It really is a triple channel DAC.
Next to each transimpedance amp there is an array of 31 NPNs: emitters grounded, bases driven in parallel, collectors connected in groups of 16, 8, 4, 2, 1. For each group there are three NPNs controlled by the logic which route the group's current either to the TIA directly, or to the TIA through an inverting Wilson mirror (3 PNPs between the TIA and the array), or to a dummy emitter follower where nothing happens. Pretty sure we have seen this kind of DAC architecture before.
By the way, I2L logic outputs are open collector so they shouldn't be able to drive the NPN current steering switches. It seems that in this chip the logic is "grounded" through a diode-connected Darlington pair, shifting its potential two diode drops above real ground. What appears to be inverter gates driving the switches, actually are ordinary emitter followers working in forward mode and feeding current from logic virtual ground into the bases of the switches.
The apparent fourth channel is another story. The TIA is a dummy - it simply outputs the same quiescent DC voltage as other amps, there is no signal current going into it. It looks like pin 17 is a reference voltage for the other outputs. There is no Wilson mirror on this channel, in its place there is a fifth opamp which drives the bases of all current sink arrays in such way to produce a particular output current from the fourth DAC. The fourth DAC is also modified, it is single-ended only and has groups of 4, 10, 4, 2, 1
By controlling this DAC, the logic forces the opamp to set different levels of current per one cell of the array. Hence gain of the main DACs can be varied.
I found only three inputs to the chip. They go to the analog section near pad 13, which is simply a bunch of comparators. Comparator outputs go to the logic, so these are digital inputs. I doubt that they are fast enough to feed complete raw data to all DACs in real time. I think it's some special purpose chip, which receives simple commands and generates more complex output. I suppose it could be all sorts of weird things, from an OSD generator to a fully integrated simple video game.
I didn't find any obvious oscillator or other clock source