Final answer:
Uterine atony is the most likely cause of the sudden gush of bleeding from the vagina in a postpartum client after childbirth.
Step-by-step explanation:
The most likely cause of the sudden gush of bleeding from the vagina in a postpartum client ambulating for the first time after giving birth three hours ago is uterine atony. Uterine atony refers to the inability of the uterus to contract properly after childbirth, leading to excessive bleeding. This condition can occur due to various factors such as prolonged labor, multiple pregnancies, and retained placental fragments.