My theory is this: 2 year olds will test the rules, what is ok and what isn't? IF you have been consistent - 3 year olds have figured out the rules, Now they will test you for continued consistency!
Parenting is all about setting ground rules, and it's MOST important in the first 4 years! If you "failed" the 2 year old test, you will go through it again in 4s and 6s and on and on until they know how you work and what pushes your buttons! Same with the 3 year old test.
One of the BEST pieces of parenting advice I ever got was to read the books "How to Talk so Kids will Listen; How to Listen so Kids will Talk" and Liberated Parents; Liberated Children" both by the same female duo of authors.
So to the question~ it is just a game for now. If you play the game, it will continue. If you don't play the game it will stop. Let your child know what is appropriate and what isn't. Give them credit! They may be young, but they GET IT! Until your point as a parent is made and proven it will hang on and resurface now and then for a while. CONSISTENCY IS THE KEY! Hang in there, it will get better, incidents will be fewer and farther between as you go.
Parenting is the most worthwhile and HARDEST job you'll ever love. In the end all we really want is for our kids to be happy, healthy, secure, responsible adults - not only respectful, but worthy of respect in return. Good Luck!