def triangle_area(a, b, c):
if a + b > c and a + c > b and b + c > a:
s = (a + b + c) / 2
area = (s * (s - a) * (s - b) * (s - c)) ** 0.5
return area
else:
return "ERROR: NO TRIANGLE"
a = int(input("Enter side a: "))
b = int(input("Enter side b: "))
c = int(input("Enter side c: "))
result = triangle_area(a, b, c)
print(result)

Пример работы программы:

Enter side a: 3
Enter side b: 4
Enter side c: 5
6.0Enter side a: 1
Enter side b: 2
Enter side c: 3
ERROR: NO TRIANGLE