Mercury, Venus, and Saturn did align in an infrequent way in December 2012.
The planets did not appear as depicted in a widely circulated online image.
Planetary alignment of Saturn, Venus, and Mercury that will take place Dec 3, 2012 is dead-on alignment with the Pyramids at Giza. Night Sky in Giza, Egypt on December 3, 2012, local time … one hour before sunrise compared with the Pyramids at Giza.
In short, there was a relatively close conjunction (about 14 degrees of separation) between the planets Mercury, Venus, and Saturn on 3 December 2012. However, a conjunction of three planets isn't a particularly rare event; these very same three planets formed a similar alignment as recently as 2007. Also, the conjunction of those three planets did not appear as neatly as depicted in the graphic above: if one were to draw a line parallel to those planets, it would be much more vertical than horizontal, and the planets were not perfectly aligned along that line (i.e., a line connecting any two of those planets would not pass through the third).
While it's possible that an observer who selected just the right time and place might have been able to produce a view that positions the three planets over the tops of the three pyramids, that view would still not have looked like the one in this idealized graphic.