how can you make ramp arc in autocad 3D?

The best and easy way is to use extrude in path command for complicated shapes

One option using 2012 is to use the LOFT command.
Another option might be to Extrude with a taper.
A third approach might involve using the Slice command and Surface option.
Many ways to skin a cat.
LOFT with PATH option definitely works.
See image in post below.
I used two profiles (one for each end).
look at this picture: http://www.cadforum.cz/forum_en/uploads/20110518_150639_Curved_ramp.jpg