If you're looking for an easy way to determine if a number is divisible by 3, then look no further than the divisibility rule. It's a shorthand way to tell if any given number can be divided by 3, without actually performing the division. Take the number in question, and if the sum off all the digits is divisible by 3, then the original number will be, too. There are four examples in this math video.
Once you learn this, move on to start learning how to divide by 2, 5 or 10.