Simpson auditioned for The Mickey Mouse Club as a child and reached the finals but not the final cut. She said she later reconnected with Timberlake and Gosling, who both starred on the show.
"Throughout life we all found our way back to each other in ways. We were in the same circuit," Simpson said. "I knew Justin Timberlake very well."
Simpson said she shared the kiss with Timberlake following her divorce from singer Nick Lachey.
"After divorce and he was out of a relationship, he was over at my house. We shared a nostalgic kiss," she said. "I thought, 'Oh, this is interesting.' He took his phone out and started typing."
Simpson said Timberlake was texting Gosling, with whom he had a longstanding bet over who would kiss Simpson first.
"He texted Ryan and said he won the bet. I was like, 'Oh, okay. So we don't kiss again? That's done,'" she said.
Simpson said she had a crush on Gosling during his Mouseketeer days.
"When I was 12, I was like, 'This guy's so cool, and he's from Canada! I don't know where that is on the map but I really think that's amazing. There was something so cute about him,'" she said.
Simpson recounts her kiss with Timberlake in her forthcoming memoir, Open Book. She also covers several serious topics, including the sexual abuse she experienced as a child.
Simpson recalled on Wednesday's episode of Today how she was sexually abused from the ages of 6 to 12 by a young girl who was a family friend.
"At the time I didn't really understand what was happening. I knew something was wrong. I knew it was wrong, what was going on," she said. "This was a very close person, and she was being abused."
Open Book is scheduled for release Feb. 4.