Texasdew, please don't "be poly" in order to fit in with a couple. If they are truly polyamorous, then at this stage they (IMNSHO) ought to be spending MUCH more time educating you than recruiting you.
I'm guessing it's going to be a closed threesome, so not so much polyamory as
polyfidelity or rather
group marriage & therefore more like "real couple + 1." The wife "wants to explore her bi side" & they've maybe already been observing that "you seem like you might be bi too."
If I was going to recommend readings & discussions for you, it wouldn't be all the usual cheery fluff about how wonderful polyamory is (& therefore how wonderful anyone is who calls themselves "poly"

), but rather on effective skills with communication & problem-solving, & how to recognize & repair an impending problem before it's a crisis.
Of course, you ought to immediately begin by reading up on
unicorns. The experience can feel like being made a couple's "special princess"... or their live-in rent-paying sex-toy, cut off from the outside world & readily replaceable. The topic comes up regularly, so there's a really long thread (made up of multiple discussions since 2009) --
Unicorns & Unicorn Hunters