I need to count up the numbers of numbers divisible by 3 in a program. I know how to figure out which numbers are divisible by 3, but not how to get the program to give me a number for the numbers that fit that distinction

In your loop add a counter that you increment esh time the divisible by 3 condition is met.

Use a for-loop and pass your increment variable to a GCD function.

Also, you should use the Euclidean GCD algorithm.