Baback,
It sounds like the problem may be that your 3DS slave session is not being
allowed to run in the background under OS/2.
Try having the slave run with MAIN-DISPLAY=VESA and if that doesn't work then
try MAIN-DISPLAY=VGA. Make sure the render output is to file only.
Also, make sure that all of the dos settings for the slave session look
condusive to background operation.
BTW, what sort of graphics card are you using?
Bailey