With what you change the p3d file to 3ds
if you have a mlod.p3d and want to export it to 3ds.
triangulate your model and send it as a .asc file to a location on your pc.
import it with a program called "med"(model editor),this can be found
by downloading the gs5 gamestudio trial version.search on google for it.
after importing the .asc file,then export it and save frame as .3ds
then open in max\milkshape or other progs.