ME3Tweaks Forums

This isn't really possible. In ME3 the exploration and combat areas must transition via a loading screen or a cutscene from what I can tell. The holster animation isn't even in the game for combat sections because of PS3 memory limitations.

If I remember correctly, the Expanded Galaxy Mod makes you able to lower your weapon through a key.
Not as good as holstering but helps maybe a bit with the atmosphere.