Change word phrase to an algebraic expression.

"the difference between three times a number and 7."

3x-7, the difference signifies subtraction between three times a number, which is 3x, x signifying the unknown number and 7. 

To understand fully the statement, we dissect each word. 

"The difference between three times a number and 7."

The difference means the operation here is subtraction.
three times the number would mean 3x where x is the number

Therefore, the algebraic expression is:

3x - 7

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