getting the 2d cords of a 3d object

Is there a way for a program to track the 2d cords of a 3d object, so my 2D hud overlay can highlight specific objects in the 3d plane.

cameraproject cam,x#,y#,z#

x,y,z = entity coordinates

followed by the use of the functions:

projectedx() x position on screen
projectedy() y position on screen
and projectedz() - is greater than zero if point is in front of camera