Answer
No, it is not.
Work Step by Step
The Pythagorean Theorem states that for any right triangle, $a^{2} + b^{2} =c^{2},$ where c is the length of the hypotenuse and a and b are the length of the other two sides. Given that the numbers are natural numbers, we plug them into the Pythagorean Theorem to see if they are a Pythagorean Triple. (Note, the longest side is the hypotenuse.)
$$1^{2} + 2^2 =3 ^{2} \\ 1+4=9 \\ 5=9$$
This is not true, so it is not a Pythagorean Triple.
