Greetings Nora,
The Donax sounds like Donax punctatostriatus Hanley.  It's the largest West
Central American species of Donax, and also one of the commonest.  It's
purple inside, and your comparison to the shape of D. carinatus sounds just
about perfect.
The Cerithium could be either C. stercusmuscarum Valenciennes or C. maculosum
Kiener.  My guess would be C. maculosum, first because I think it looks more
like C. litteratum than C. stercusmuscarum does, and secondly because it is
the larger of the two.  C. stercusmuscarum, like C. litteratum, is usually
about 1 inch, but C. maculosum can reach almost twice that size.  They are
both common.
As for the Astraea - There is a little West Central American top shell called
Astraea spirata (Dall) that is very similar to A. tecta americana.  I don't
think it is too common though.  If not that, then probably A. gibberosa
Dillwyn, but that species is usually not as tall-spired as A. tecta americana.
Regards,
Paul M.