Connie had no prior knowledge that Fredo was going to be killed. It was she who begged Michael to forgive Fredo. It's possible--even likely--that Connie figured out later that Michael had Fredo killed. Some people here believe that the scene in III where she said he "drowned" actually was a veiled way of Connie telling Michael that she knew he had Fredo killed--and that she forgave Michael.

Michael almost certainly didn't tell Tom (or anyone other than Neri) that he was going to whack Fredo. But Tom probably would have figured that Fredo was a dead man from the moment Michael returned from Havana. He saw what happened to Carlo. In fact, in the novel, he explains to Kay that a traitor can never be forgiven because he will be a danger to the family forever. He argued against Michael killing Roth and the Rosatos because they were "on the run"--not a danger to Michael anymore, to Tom's way of thinking.