import math


def area_triangle_sss(side1, side2, side3):
    """
    Returns the area of a triangle, given the lengths of
    its three sides.
    """
    
    # Use Heron's formula
    semiperim = (side1 + side2 + side3) / 2.0
    return math.sqrt(semiperim *
                     (semiperim - side1) *
                     (semiperim - side2) * 
                     (semiperim - side3))