The first Terminator, seen below, was unveiled in the late 1990s.
The main difference between the first and second Terminators is that the first came with two hull-mounted 30 mm automatic grenade launchers in addition to all the other weapons. The first version also had a more expensive T-72 tank hull, and a turret with the Ataka missiles mounted vertically.
While the Terminator 2 is lighter, cheaper and easier to operate than the first Terminator, it also has less firepower and can only fire at one target at a time, while the first could fire at three targets at a time.
Photo Credit: Vitaly Kuzmin
In any event, the Terminator 2 that was spotted in Syria, seen below with President Assad, is reportedly the only one Russia has that is battlefield ready.
And the fact that Russia only has one Terminator 2, and seems to have had trouble selling, one can only conjecture as to why Moscow is building a third version.
Below is the most recent Terminator, the Terminator 3 or T-15, which is part of the new Armata Universal Combat Platform series.
These Armata vehicles, which includes the T-14 tank, are still prototypes and have yet to be mass produced. As such, it's still unclear which vehicles are part of the Armata series, and what the capabilities of the T-15 or Terminator 3 are. Nevertheless, if this T-15 truly is the third Terminator, it appears to only have one 30mm cannon.
Source: Sim Tack, chief military analyst at Force Analysis, and former Stratfor analyst.