The thing about poly is that it's about loving, real, important relationships. I get that that wasn't the "plan" with Sam but it's what you've got and you'd do well to honor it. That means, to my mind, letting it grow at its own pace. Not getting sucked into NRE and forgetting about Alex, by any means, but also not letting her entirely dictate what pace is ok for *your* feelings... I mean, how must that feel for Sam?
Do. Not. Lie. To. Alex. You'll be violating both her trust and Sam's by saying you're in an honest, open relationship and then not living that way. Do the brave thing, the thing that will ultimately strengthen your relationships rather than weaken them, and just communicate!
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.