For me Kate is the only one of the three who could be H or involved.
Ted has built a career on catching bent coppers, if he is bent it makes a mockery of everything he has stood for and the audience connect to him for.
Steve joined AC-12 somewhat reluctantly after refusing to lie in his statement. He has been thrown down stairs, left with crippling back pain, drug addiction and can't get it up. He's lost basically everything.
Kate, while she has lost her family life, due to her affair. She has otherwise kept herself fairly clean, she hasn't lost as much. Hasn't suffered like the other two. She is the only one I could see it maybe being her. But I still hope it isn't her, and it's someone else impactful.
So Steve and Kate can finally get over their sexual tension and calling each other mate all the time, Steve takes some little blue tablets and they finally get it on.