Speculating on 3.12 off the spoilers and episode descriptions:
"I don't want to be A woman you love." is said (Tatsu saying this to Maseo in present day is my second choice but...)
Oliver returns at the end of the episode
Felicity is Mad. As. Hell.
Also, a 3rd party, not Palmer, comes between Oliver and Felicity.
I think it's Thea that Oliver and Felicity fight over. If I were Felicity and I believed Oliver was dead, then I would consider it my responsibility to protect Thea. Especially since Oliver died trying to do just that. Wouldn't the idea that Thea is still in the dark, in danger, and at the mercy of her lying, drug-murder inducing dad weigh on Felicity?
If Felicity does reveal to Thea even part of Oliver's secrets or that Oliver died (again) that will be a big issue for Oliver when he returns. Since 3.12 is described as Team Arrow arguing about whether or not they should work with Merlyn, it's possible that Felicity might take it upon herself to talk to Thea. She is dead set against Merlyn breathing, let alone working with him. Thea is very much at risk and keeping her in the dark at this point is dangerous. Felicity would know that. Also, Thea deserves to know the truth about things.
However, when Felicity explains her reasoning to Oliver, he will probably not like hearing that Felicity told Thea anything. Oliver is so protective of Thea and wants to keep her pure/kind. Felicity may have reached her limit on Oliver making decisions for everyone and controlling what everyone gets to know, including Thea. Throw in his opinion on Laurel's new activities. (I love Oliver but things need to be his way.) Felicity will tell him "If that's how you treat the women you love, then I don't want to be a woman you love." Felicity revealing to Thea things Oliver doesn't want Thea to know would be a big conflict. I can't see either of them giving in on it.
Depending on how much Felicity does tell Thea, Thea could ignore her or not believe her. Thea ignored/disregarded Oliver on the subject of Merlyn too.
Roy will take over warning Thea about Merlyn in episode 13. Felicity would back off now due to their fight and because Oliver is alive so Thea is his responsibility again.
