The perimeter of a triangle is 90 cm, which is equal to its sides if their ratio is 3: 3: 4.

1. Since the ratio of the sides in a triangle is known, the path of one part is equal to x, then we get the value of the perimeter:
3x + 3x + 4x = 90,
10x = 90,
x = 90/10,
x = 9 cm.
2. Find the sides of the triangle by multiplying the length of one part by the corresponding number of parts of each side:
1 side = 3 * 9 = 27 cm.
2 side = 3 * 9 = 27 cm.
3rd side = 4 * 9 = 36 cm.
Answer: the sides of the triangle are 27 cm, 27 cm and 36 cm.