This question How and when did some European languages acquire retroflex d and t? makes me curious: Are there any languages that have three different kinds of d's (or t's or n's or s's) exhibiting a place of articulation contrast dental–alveolar–retroflex?
Certainly. There are many Australian languages with a lot of coronal POAs for stops; dental, alveolar, and retroflex stops are generally differentiated, in addition to a palatal/alveolo-palatal series. This was apparently historically true of Tamil, although not so much for the modern language. I believe it also occurs in other Dravidian languages.
Australian languages rarely have sibilants (and those that do don't seem to contrast many POAs for them). I don't know of any languages with a straight-up dental–alveolar–retroflex contrast for sibilants. However, Basque contrasts lamino-dental sibilants, apico-alveolar sibilants, and palatal/postalveolar sibilants.
Proto-Dravidian is reconstructed with three coronal stops - laminal dental, apical alveolar and retroflex. This alveolar stop phoneme is one of the most unstable in the family. Old Tamil did preserve all three of the coronal consonants, but most dialects of Modern Indian Tamil have lost the alveolar stop, having merged it with the alveolar tap when it appears singularly, and with the dental stop when it occurs geminated. However, certain Indian dialects of Tamil, Sri Lankan Tamil, and Malayalam retain the alveolar stop in post-nasal and geminated position. Further several Dravidian languages of the tribes living in the Nilgiri hills of southern India (Toda, Kota, and Irula included among them) do retain the three-fold coronal consonant contrast that they've inherited from Proto-Dravidian.