STA form needs to be completed at least 3 days prior to departure
Nope nope nope. It is
recommended that you complete it no later than 72 hours prior to departure in case you get rejected and have to get in touch with the Embassy. The only requirement is that you get it done before check in - you can't check in without it done.
However! I recommend you get it done now. Three reasons - firstly its valid for 2 years. Secondly - it's out the way, one less thing to worry about nearer the time. Thirdly - the US government are considering charging a fee to obtain an ESTA. For the moment, it is 100% free!