Read three marks and show their average, rounded to two places…

Plan. Add the three marks, divide by 3, and round.

1. Read the marks with float(input(...)).
2. average = (m1 + m2 + m3) / 3. The brackets make the addition come first.
3. $(72 + 85 + 90) \div 3 = 247 \div 3 = 82.33\ldots$

Answer: $82.33$.