Epihany. having a separate function for calculating area makes the code much simpler to understand and implement...

float calcArea (triangle tr) //func for cleaner code to calculate the area

{ float p = (tr.a + tr.b + tr.c) / 2.0;

return sqrt (p * (p - tr.a) * (p - tr.b) * (p - tr.c));

}

void sort_by_area(triangle* tr, int n)

{

for (int i = 0; i < n - 1; i++)

{

    for (int j = 0; j - n - 1; j++)

    {

        if(calcArea(tr[j]) > calcArea(tr[j+1]))

        {

            triangle temp = tr[j];

            tr[j] = tr[j+1];

            tr[j+1] = temp;

        }

    }

}

}