Adding More than Two Fractions

Im just reading through a basic pre-algebra book; however, I am not certain what the author is trying to get across here.

Text preceding the problem: Finding the LCD for three or more fractions is pretty much the same as finding the LCD for two fractions. Factor each denominator into its prime factorization and list the primes that appear in each. Divide the LCD by each denominator. Multiply each fraction by this number over itself.

Example

$\displaystyle \frac{3}{10} + \frac{5}{12} + \frac{1}{18} $

The Author then shows the prime factorization of each denominator

10 =** 2 * 5**

12 = **2 *** 2 *** 3**

18 = 2 * **3 *** 3

LCD = 2 * 2 * 3 * 3 * 5 = 180

I will stop the explanation here. I understand doing the prime factorization of each fractions denominator. But as to which numbers from the prime factorization to use (the bolded ones) is beyond me could some one please clarify how one is supposed to know which numbers to multiply together to result in 180?

Many thanks