Please talk to your pediatrician. Express your concerns - a phone call or an appointment on your own (without kids, or at least only w/your oldest) might be easier. It absolutely could be that there's a particular parenting technique that might help, it could be that he's in a challenging, boundary-pushing phase - or there could be something larger going on for which interventions might be appropriate and useful. If interventions are appropriate, the earlier you start, the faster the interventions work. But let your pediatrician help you, it's part of their job to help you figure things like this out.
Rambunctious is normal for a 3 year old. Energetic is normal. Even inappropriate (mean, violent, impulsive, etc) is sometimes normal. Hurting your siblings is not acceptable. Disregard for consequences is quite worrisome.
My 6-1/2 year old son was diagnosed with ADHD over this past year, and while he's not particularly oppositional (as it sounds like your son might be), that kind of behavior is very typical of ADHD. My son's preschool teachers first started raising red flags when he was 3, although just in the "lets keep an eye on this" realm. When he was 5, they said: it's time to have him evaluated. We went through extensive, thorough evaluations (audiology, opthamology, psychology, sensory/occupational therapy eval) and the results were very consistent: ADHD. I see you're also in St. Paul - we went to Gillette, they were amazing, Dr. Raymond Tervo.
Your first step should be your pediatrician. The fact that you describe your son as "loving, sensitive, smart" and "dangerous" and "I'm at my wits end" suggests to me that there's something going on inside his brain/body that he can't control. It could be ADHD, it could be any number of other things, but you don't have to figure that out for yourself. Talk to your pediatrician.