The identical Dahm triplets Erica, Jaclyn and Nicole were born on December 12, 1977. They were featured as Playmates in December 1998's Playboy magazine. There have been no other triplets appearing as Playmates, but twins appeared thrice:

It must be noted, that triplets are quite well represented in Playboy, since they have appeared in 1 out of 672 issues (0.15%), compared with 0.18% triplet birth rate. Twins, on the other hand, have appeared only in three issues (0.45%), six times less than the 2.9% rate of twin births. :-( To stop the discrimination and to compensate all Playboy "readers" for all these years, Playboy has to dedicate the whole 2003 and half of the 2004 to twin Playmates. :-)

Thanks to haze for pointing out that I missed Bernaola twins (shame on me).