The three barrel Webers were supplied for Ferrari 512BB, 365 GT4/BB and for the Lamborghini Miura P400 as well as for the Porsche production cars. Model number for the Ferraris was 40IF3C and 40IDL3C for the Lamborghinis. I currently have a set of Webers from a 512BB and they have date codes. It is also interesting that the Ferrari carbs utilize some of the extraneous bosses found on the later castings. These bosses were used for extra items such as cold starting features and for mounting various components we Porsche guys don't use. The picture is of a 40IF3C throttle body. In 1981 the 512BB began using CIS so I expect Weber saw little use for making the three choke carbs and thereafter sold the design to Carter carbs here in good old USofA.
I believe the three choke Weber was developed for a Lancia V-6 in the mid 1950s but haven't been able to pin this down; I have found that the 1961 Ferrari (Dino 156 "Sharknose") Phil Hill won his F1 championship driving used the three choke Webers. Since the Solexes were causing issues for Porsche and the Webers were an off-the-shelf Weber product it makes since that Porsche went with them.