Who's This? The three guys in fatigues on page 8 of Who's Who vol.XV.
How you could have heard of them: They haven't appeared since, but those comics aren't that old.
Example story: G.I. Combat #282-283 (1986) by Robert Kanigher and the great Sam Glanzman (the MVP in these stories)
The Mercenaries' adventures were a little more down-to-Earth normally, but had a high incidence of them failing and getting help from an outside agent. The other story in #293, for example, has a volcano interceding on their behalf, basically.
Who else? Before page 8, there are some villains I haven't actually seen in action, but neither Matrix-Prime nor Matter Master inspire me. Quite the opposite. No, we're forging ahead with some good old Golden Age content next time.