3240 = 2^3 x 3^4 x 5; 3600 = 2^4 x 3^2 x 5^2; H.C.F. = 36 = 2^2 x 3^2
Since H.C.F. is the product of lowest powers of common factors,
so the third number must have (2^2 x 3^2) as its factors
Since L.C.M. is the product of highest powers of common prime factors,
so the third number must have 3^5 and 7^2 as its factors.
Third number = 2^2 x 3^5 x 7^2