It's a toss up on both questions.
The 8 and 7 year old might make the height requirements for the thrill rides, the 5 year old will not. You may have to split up if this is the case for the rides at IOA because they'll want to go on Hulk and Dueling Dragons if they can.
And as to spending 3 days, again that's a toss up. Depends on how much money you have budgeted. You'll have your work cut out for you "dragging" 3 kids around. Some people have kids that can keep up. But after a few hours, they'll get tired, and you may have to take them back to the hotel for a rest. This happens with my daughter and I. After a few hours, our feet get tired and we need a rest. Then I feel like I'm dragging her because she can't keep up with me. Little kids can't walk as fast as adults. When my daughter was 8, I used to push her in a stroller. If they made the strollers bigger, we'd use them. She doesn't fit anymore. There's a marketing idea for the parks, super sized strollers.
As to what park to do first, it seems that most do Disney first, then HRH.
At least you'll have your magic hotel key so you can get thru the lines fast.