Is it true that after a cesarean section for triplets, it is impossible to flatten the tummy?

Nope. Your abdominal wall will have been stretched, but can recover almost completely with regular exercise and weight loss >6-52 weeks after your cesarean section.
Never say impossible. The issue brings up questions of motivation, difficulty, body image etc. Although a pre-pregnancy figure is difficult, I have had jazercise teachers deliver & appear back in shape within a month. With 3 it would take more time & effort & some would accept the change as a badge of honor. Electronic muscle stimulators can help tone the belly muscles & help rehab that area without painful sit ups.