Warning: SPOILERS
Here’s my take on this movie.
Tully was a person that Marlo created in her own psyche to become a better mum and wife.
There was no threesome. She was simply following the fantasy that (imaginary) Tully implanted in her head.
When she crashes and the car goes down, Marlo is the only one in the car. When she is in the hospital and the receptionist asks her husband for his wife’s last name, it was TULLY.
The psychiatrist in the hospital is clearly concerned about Marlo’s mental state and expresses this to Marlo’s husband. This also convinced me that Tully was a made up “miracle worker” that somehow kept her somewhat sane and able to handle the MANY obstacles that come with motherhood and marriages.
She finally finds closure at the end as she says “goodbye” to the friend that she created in her own mind because she finally realizes that she can handle these hurdles on her own.