  The conformal extensions of three kinds of special relativity with
ISO(1,3)/SO(1,4)/SO(2,3) invariance on Mink/dS/AdS space, respectively, are
realized on an SO(2,4)/Z_2 invariant projective null cone [N] as the
(projective) boundary of the 5-d AdS-space. The relations among the conformal
Mink/dS/AdS spaces, the motions of light signals and the conformal field
theories on them can be given. Thus, there should be a triality for these
conformal issues and the conjectured AdS/CFT correspondence.
