It sounds like she can't deal with jealousy and NRE.
They seem to have unrealistic expectations: that two people can fall in love with a third to exactly the same level, at exactly the same rate. However, it just doesn't work that way.
This couple has a lot of learning to do about polyamory before they'll be ready for what they're trying to accomplish. It's up to you whether you want to be toyed with and taken for granted while they figure that out. I personally wouldn't.
Besides... you're a woman who wants to date a couple? The world is your oyster.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker