There's actually lively debate about whether or not it counts as poly if someone is in the dark, so you're in good company not being sure. I'd say you and your wife are practicing poly while the Ladyfriend (LF) perhaps is not, but some would insist the whole thing is just cheating.
Do you know why she doesn't want to come clean to her husband? Personally, I probably wouldn't choose to be in such a relationship for a couple of reasons, including "if she's lying to one intimate partner she'll lie to another" and "what if the husband finds out, goes crazy with rage and comes after me?". But I know these things can be complicated.
Some thoughts, from the perspective of someone in the LF's position if your wife does get involved too: http://www.morethantwo.com/coupledating.html
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.