Can 18 24 30 be the side lengths of a right triangle?

YES.

18 and 24 are the two leg lengths and 30 is the hypotenuse then by Pythagoras' Theorem :- 182 + 242 = 302

324 + 576 = 900......which is true and therefore the three side lengths 18, 24 and 30 do form the sides of a right-angled triangle.