Vivian,
Troubles refer to deaths, accidents, or bad events in general.
Three is considered a magical number in many cultures
"Trouble sires three children" is another way of stating a common Western superstition that bad things happen in threes.
"Trouble comes in threes", or "Bad luck comes in threes" are others ways of saying it.
Some people actually believe that if something bad happens then two more unlucky things will happen along with it. Do you? :)
In some cultures the number three is a lucky number!