Howard Joseph Sorensen notes in an entry in Volume 10 of the New Catholic Encyclopedia that the number three indicates completeness and is superlative—a thing is entirely what it is said to be. A person dead three days is really dead. One who is thrice holy is perfectly holy.
Seven also indicates completeness, and can be used of good or evil. Mary Magdalene, for instance, was possessed by seven devils.
In the end Sorensen makes the important statement that the Bible never attributes a special power to numbers and a number has no special meaning apart from the thing signified.