I am having a hard time diciplining my 3yr old when she is having a temper-tantrum, not listening or being rude to me & others. I've tried everything?

Temper tantrum. Here are some ideas to try. 1) time out 2) remove your child from the scene and discuss why he or she is upset 3) take away favorite toy-- child earns it back with good behavior 4) reward method-- good behavior earns something (not food) 5) teach child ways to deal with stress besides temper tantrum (coloring, writing, favorite book, running outside, etc). Good luck.
Positive Rewards. Start a behavior chart at home. I like the ones that have the little magnets and you can customize the days of the week. Pick the behaviors you want like the " no whining" magnet "no tantrums" or "be respectful" magnet with perhaps another one or two "chores". Three behaviors are good for age three. Then she gets a + magnet for each day of the week that she does well.
Tantrums in kids. Check out this book on kindle: parenting the kids according to a child psychiatrist. Good luck.
Don't let her engage. You in an argument, ever. While you're both calm, tell her what behaviors you want ; the consequences for not doing so. When she tests you, "walk, don't talk" her to time-out as many times as she gets out + 1, then set the timer for 3 minutes. Ignore all protests during the "escalation" period that precedes "extinction" of any behavior. Read "little people" by dr. Ed christopherson (on amazon).
Tantrums. Check out this book on kindle: parenting the kids according to a child psychiatrist. Good luck.
Behavior probs. I suggest you read: parenting the kids according to a child psychiatrist. It's on kindle for $5.95 and will tell you exactly what to do with this and many other issues you will face with your child.