...Is it possible to daisy chain the three units and connect the antennas to the front of the first unit...
Not to "the front" connections of a G90, which would need to be "dialled up" as "Ant In: C & D" in the set-up menu, but would still NOT provide the DC power that's required by the cable-compensating "head" amplifiers built into Line 6's paddle antennae.
Selecting and connecting to the REAR "A & B" inputs of your "first" G90, then using the supplied, short, LMR-195 antenna "patch" cables from that receiver's "B/C Out" connectors to the rear-panel "A & B" connectors of your identically set-up "second" G90 ...and finally repeating that procedure from that G90's "B/C Out" pair to the "Antenna A" and "Antenna B" connectors on the back of your XD-V75 receiver ...should, however, work really well.
(Incidentally, that's the "electrical order" I'd recommend for hooking up our "daisy chain", regardless of the actual "physical order" or "place" that those three receivers occupy in our rack.)
...Are there pros / cons using the XD-AD8?...
When fewer than four receivers are involved, I mainly see three "cons", expense, complication and bulk, ...the one "pro" being the elimination of the need to accommodate three "wall wart" plugpacks and their associated power board inside our rack.