After 3rd csection was told that my uterus is thinned and i should get permanent birth control. what are the symptoms and possible causes?

Second opinion: They are probably referring to the site of your prior c-section incisions when they refer to the thinning of your uterus. Yes, there are increased risks with more c-sections, but before you do something permanent and regret it later, get a second opinion with another ob/gyn.

Multiple cesareans: Multiple surgeries, big baby can cause thinning.

Asymptomatic : Once healed the uterus will be dormant and no symptoms. If pregnant and thinned, can rupture with labor. Increased risk but not 100%.