Hello Amy
Confusingly, the same question seems to have been posted by a Sandie almost at the same time as you posted your question.  There must be something in the air (or, probably more mundanely, there’s a glitch in our system). Hopefully my response will help you both.
That said, there isn’t an exact answer to your question. If you are worried that you’re trying to pack too much in to your opening three chapters, I suspect that’s exactly what you are doing.  Instinct is a good thing to listen to when writing.  The long and the short of it – no pun intended – is that what you should be aiming for are gripping chapters that keep the reader engrossed and wanting to turn the page.  Some people do this by using very short chapters (Dan Browne is one who springs to mind). Others do so by ending each chapter on a cliff hanger (exactly what Jed Mercurio did so well at the end of every episode in the ‘Line of Duty’ series on BBC TV).  Other writers set up characters with whom we immediately want to spend more time. Some go for a really big, exciting opening and keep up a relentless pace.  There are lots of ways to ensure a compelling opening without cramming ‘all the best parts’ in to the first three chapters of a novel.
The other thing to bear in mind is that you want to use your opening chapters to move an agent (presumably, in the first instance?) to call in your whole manuscript.  You therefore most definitely don’t want to cram ‘all the best parts’ of your book in to the opening chapters.  You want that agent to be gripped by the whole thing.  So don’t overwhelm yourself – or your book or your reader.  Think about pace, about hooking and then keeping your reader and about how your plot is put together and how it unfolds as your reader turns the page.
If you feel stuck until you know about chapter length, pop into your local bookshop and skim the books that are selling well. That will give you an indication (though possibly not a consistent answer).  As a rule of thumb, for MG, I’d be inclined to err on the side of brevity.  Kids are often reading after a long, tough day and sometimes just relish a satisfying, short read before bed.  A bit like the agents you’ll be hoping to hook, in fact!