There's a conv3ds.exe .3ds to .x conversion program floating around somewhere (did I hear it comes with DarkBasic or DarkMatter?).
JTEdit (
http://www.jtgame.com/jtedit) supports reading and writing both .3ds and .x files with animations, though I've no guarantees it works for all .3ds files.
But if you're modeling in Max, the .x exporter is the best bet. That is, if it works. The one I tried from the DirectX SDK a long time ago didn't work well for me, but maybe they've fixed it.
-John
John Thompson
http://www.jtgame.com/jtedit