i asked a simialr question a while back
http://www.disboards.com/showthread.php?t=2226798
we also arrive on the 3rd. after a 9hr flight and 2 young children, we are taking the friday off to let them recover; and spending the day by the pool/exploring the hotel.
we are going to aquatica on sat. probably for the whole day (looking at getting the all you can eat all day pass).
Seaworld on sunday. park is open until 10pm (goes down to 5/6pm the week after). hoping that the extra hrs will balance out the extra crowds, and make the park manageable (does that make sense?).
as we are planning to stay at seaworld all day on sunday, we'll take monday to rest before hitting disney on tues.
i dont know which day would be best for you to do MK. Mondays are meant to be one of the busiest days of the week, and with it being labor day, it may be best to avoid it. it also depends on whether seeing spectro & wishes is important to you (spectro showing sat and sun; wishes showing sat, sun, mon while your there), and plan around that.