Thanks for asking! I don't recall any serious discussion of this between me and Metagaming (i.e., Howard Thompson commissioning a third game, which I would've been happy to produce) and Metagaming ceased operations pretty suddenly. Would have been easy enough to do: more map, more units, maybe a more elaborate electronic warfare module.
Wow, how'd I miss this. I'd love to hear your thoughts on what you'd have included in a 3rd game in the series, as well as what you'd include if you were to produce the game today. Say, Helltank Returns?