I'm still working on getting proper electrical hookup for my kiln, so can't speak from personal experience - but I've done a lot of reading in anticipation of troubleshooting such issues when I do get it plugged in ...
It seems that, even in an otherwise empty kiln, if your sitter is shutting off with a 3, but you're not even bending all of the 2's, your sitter may not be adjusted correctly.
What did the '3' on your sitter look like when it shut off ? Below is a pic from a Dawson kiln-sitter manual. If your sitter cone looks more like the one on the left... it could suggest that the "sensing rod" is either bent, or out of adjustment - causing the sitter to trip prematurely. (A bent rod could also stick - causing it to trip late.)
You said it has 5 switches - are they all simple on/off switches, or do they have high/low options? Doing a 'staggered start' (not turning all on at the same time) - or, if they're adjustable (high/med/low), not setting all to the same level - might help you balance the temps.
sitter1.jpg 43.93KB 1 downloads
Also... any chance you accidentally put a 2 on the sitter for your test, instead of a 3 ? (Sorry, my tech-support background makes me ask those 'is it plugged in' sort of questions.)