Let's see by using the Pythagorean theorem.
3 and 4 are legs. 3 is the shortest leg.
7 is the hypotenuse.
The theorem states that:
a² + b² = c²
a = 3
b = 4
c = 7
3² + 4² = 7²
9 + 16 = 49
25 = 49 (Incorrect)
So it is not possible for 3,4,7 to be a right triangle.