I changed the fluid in my 3830 a few months ago.
I got a few 5 gal buckets and opened the drain holes one after the other as quickly as possible. It seems like the drain hole closest to the front of the tractor drained the most fluid. It could be because I opened that hole first /forums/images/graemlins/confused.gif? If my memory hasn't completely failed me (I just turned 40 /forums/images/graemlins/crazy.gif) the front hole drained more than 5 gallons.
I suggest keeping extra empty containers around.
Better yet - get a 20 gallon trough.