Parenting 3 young children is very stressful, hard work. Everyone loses it sometimes. You are a good parent obviously, in that you really want to do the best for your kids and are working on it. There are lots of things that can help you make more progress. If you haven't done so already, read my book "Raise Your Kids without Raising Your Voice" or download the MP3's on the subject on this site www.parenting-advice.net.  Take a parenting class in your area or on-line to get more options and more support from other parents. See a professional parenting consultant, anger management specialist or mental health professional who helps parents. See a naturopath for help with stress reduction. These avenues will lead to deep, long-term improvement. In the short run, a quick route to stopping your yelling behavior is to punish yourself for yelling each time you do it. For instance, after each yelling episode, write out 2 lined pages of "I calm myself down before I deal with the kids" or something like that. If you're still yelling after 3 days of this writing, add a third page of lines for each episode. Three days after that, add a fourth page. Unless you want to be spending your life writing out lines, you'll find that you have soon cut the neural pathway in your brain that leads to yelling.  Good luck with this - you are bound to succeed if you apply yourself.   Sarah Chana

I've been a nanny for over ten years and I've had remarkable success with 1-2-3 Magic.  I currently work with 7 year old twins and they responded immediately to this method of parenting.  It's a quick read, you could probably get it at the library.  You, your husband, and any other in home caregivers (Grandma, Nanny, Babysitters)  would need to read it and use the technique.  It teaches that kids get a huge sense of power from being able to upset an adult and by taking that away and maintaining calm assertive parenting things being running smoothly almost immediately. 
It's also very important that you and your husband discuss any disagreements over how parenting is being handled privately away from your children's ears.  The kids should see a unbreakable "What Daddy says goes." attitude from Mommy and "What Mommy says goes." from Daddy.  The same support should be given to any other care givers in your home.  It gives  each  adult the authority the need to discipline effectively.  Things will fall apart quickly if you don't present a united front.   I hope this helps.  :)  Caroline