From Croteam FB Page - Quick update on Xbox 360 Serious Sam 3: BFE regarding questions is it going to be retail or digital distribution - it will be released digitally on Xbox Live Arcade (XBLA).
http://www.facebook.com/pages/Croteam/191676850855803
AND
Regarding Xbox 360 SS 3, it is in development, so we are on it since PC was released... It does require time to do all kinds of optimizations, first to fit all in memory and to get decent framarate, as 360 is rather old hardware now (512MB RAM, lower range graphics card etc).
Please be patient for around 4 more months for 360, we are doing our best to finish it in that timeframe.