Some wise person must have pontificated in years gone by that "troubles always come in threes" - and we've been quoting him or her ever since.
When I was growing up, as soon as one bad thing happened, I waited in fear and trembling for the other two problems to occur. My dad used to use the expression "waiting for the other shoe to drop" because he said that every night when my mother came to bed, she took off one shoe and let it fall to the wood floor, and although he lay there waiting, he never heard the other shoe. Either she realized the noise might be disturbing and corrected her error with the second shoe or she went to sleep with one shoe on and one shoe off.
I remember going to bed one night when I was of elementary school age, and feeling that I had the troubles of the world heaped upon my head. As I recall, there were indeed three major concerns: my sisters were getting to go to my grandmother's house in Ohio and they thought I was too little to go with them; I was embroidering a sampler and had become stuck on French knots, a needlework stitch that simply baffled me; and lastly, I had broken my Mickey Mouse watch and was scared to tell my mother.