Vertical indent(seems like a hollow) 2 3 inches above navel, had a c sec 3 weeks back, suggest what can it be?

Diastasis recti?: If after pregnancy, there is a space between the frontal abdominal muscles, it may be "diastasis recti" - described as an increase in the distance between the right and left rectus abdominis muscles, created by the widening of the linea alba which connects the two rectus abdominis muscles. Sometimes, it's called diastasis rectus abdominis. The Ob-Gyn doctor can check to see if this if the problem.