A motif is any recurring element that has symbolic significance in a story.
According to this definition, the motifs that can be found in "The Story of the Three Little Pigs" are the lines said by the wolf at the door of every pig's house:
"Little pig, little pig, let me come in.'
'No, no, by the hair of my chiny chin chin'
Then I'll huff, and I'll puff, and I'll blow your house in."