From the Area 51 progress tab, it is apparent that there are 13 current beta sites, 12 of which are public.
http://stackexchange.com/ links to SO, SF, and SU, MSO, Stack apps, and the 9 oldest public beta sites.
This appears to be a bug - the list should include every public beta, and has not been updated to include the new three in the past 6 days.
The complete list is present at the bottom of every beta site.
If this is not a bug, please change the tag to status-bydesign and answer the question which logically follows:
What is the criteria for a public beta to become a member of the list at http://stackexchange.com?
The listed sites are older, have more questions/amswers, and have more users than the three (Tex, GIS, and Home Improvement) which are not. However, there is no clear distinction between the two groups.