If you declare 3 variables as num1, num2, and num3, 3 separate locations in memory are allocated. These are called scalar variables.
Instead you could declare num to be an array with a size of 3. The elements of an array are indexed, starting with 0. You would refer to these values as num[0], num[1], and num[3]. The index can be a variable. For example if i=2, then num[i] would be the same as num[2]. This makes it easy to use loops to process the array.
Arrays