Just to give you some kind of picture: I have a serious medical condition, as does my oldest, and both of my kids are twice exceptional, not just gifted. I was also a military wife, which is fairly demanding, and I went to college part-time and intermittently.
When my oldest was twelve, I realized that he argued so much because he was bored. At that point, I began redirecting his attention instead of arguing and life got vastly better and he quit making me crazy.
I was still active on parenting lists and that idea was a big revelation for a number of parents who indicated that announcements that "I'm bored" tended to precede problematic social behavior.
"Idle hands are the devil's workshop" for bright kids. Keeping them occupied is the best way to keep the peace.
Not to diminish the work you did, but did you manage to do that along the full time work, or did you have to stop or reduce work hours?
Not to mention that this 3yo is happy to sleep for 7-8h every other night because there's so much stuff to do. :)