Assuming you know all three lengths, a, b, and c. The semi-perimeter (1/2 of the perimeter) of the triangle is s. Knowing that, you may determine the area based on these calculations:
s = (a + b + c) / 2 or 1/2 of the perimeter of the triangle
A = SquareRoot(s * (s - a) * (s - b) * (s - c))
Enter the length of all three sides. What the unit of measurement is exactly is not important.