The term 'PUT NUMBERS TOGETHER' can be solved by the three-letter word 'ADD'. This term is used to describe the mathematical operation of combining two or more numbers together to get a total sum. For example, if you ADD 2 and 3 together, the result would be 5. Adding numbers together is a fundamental part of mathematics and is used in many different applications, such as calculating the cost of items, determining the area of shapes, and solving equations.