To solve the average of three numbers a, b, and c, use the formula (a + b + c) / 3, as demonstrated with the example (3 + 4 + 8) / 3 = 5. For a general case with n numbers in an array, the average can be calculated by initializing a sum variable N to 0 and iterating through the array to accumulate the total. After the loop, the average is obtained by dividing the total sum N by n. Specifically, for n = 3, the process involves adding each element of the array to N and then dividing by 3. This method effectively calculates the average for any set of numbers.