Have you contacted the public schools yet? (are you in the USA??) If her behavior is having an impact that is causing social delays she can qualify for early intervention and then you can have her in preschool classes (even with transportation if you want) for free. They can develop social goals (you are part of that). Just a thought. I think it is pretty hard to just treat one diagnosis and not affect another. but I am understanding that you are saying they are not officially calling it adhd or using traditional medications for adhd and just trying to help out with the behavioral challenges? I hope I am understanding it right, anyway...
One thing I did with my son when that little (besides that he was in every therapy I could get him in and also in Special Education) was to change how I consequenced things. He did not learn as other kids learned. I got the book The Explosive Child and tried to apply it to a younger child like this the best I could. I also used 123 magic and it actually did help a little bit. Much of what helped my son was doing sensory things like when he was really not cooperating that was the time to roll him up in a blanket (we called it make a hotdog, some say taco etc...) and have the blankets give pressure, we also put weighted blankets on him. I had a big red ball (one of those exercise balls) and would sit him on my lap and bounce him at night every night for years. Did it during the day when he needed calming. He had one chair in our house that was perfect for him to rock in and bounce his back into (had a fluffy back) so he could get that deep pressure feeling of smashing his body into it.
I had to teach my family that we might be very late for things and we were not being rude, transitions take longer for us. In fact we may need to cancel at the last minute or leave early. Sometimes I turn my car around on the way. Family and friends just had to learn to understand.
I used a lot of pictures and schedules. So for example if we were going to McD's for the play land with cousins, I had a "rule" sheet (didnt' call it that) that showed a picture of be quiet, or our symbol for hands to self, or sit down etc.... if there were issues I would not get into a big talking match, would just show him the picture....my family is still amazed how much that helped (people tend to think non verbal kids need that only but not true, kids who get overwhelmed and really unable to attend do well with this too, especially if they are more visual learner types)
UMMM...lets see (I'm reaching back).... We also had a wallet filled with different pictures of my son in different moods. we had talked about them often. THEN when he was upset or too silly or whatever, we could use those pictures to help communicate.
Sometimes he woudl grab it to say how he felt, sometimes I used it to say Q is angry...etc.
Much of what I learned was to NOT TALK, threaten (if you dont do X then no Y...causes big meltdown), breathe, and lots and lots of redirection. People even now will say well he needs a meaningful consequence. THEN he will learn. OK if that was true he would have learned LONG LONG ago. but not saying no consequences, I just never never do it in the middle of a rage or other oppositional behavior. First goal is to get back into a "ready" state... ready to sit, ready to listen, ready to eat, ready to play...whatever that state of a "good place" is for your child. Once there for a while you can review the situation. If your child is too young to remember that far back you might have to let things go but if you have a broken toy right there to show, since you broke brothers toy....X will happen (whatever is appropriate for the developmental level of your child).
I am not saying these will work for you but you asked what others have done and I am trying to remember some actual real things I did, not what books or counselors told me to do, lol.
I will try to think more....