Multiple Buck Converters
All the inputs of your converters can be connected to the 12V power source. There is no need for diodes.

If you want some clarity about additional diodes that others have used then please provide links to that information. They may be doing it with multiple power sources feeding different converters to prevent power going back into a turned off supply. (which is not what you are doing) 

Edit-
The link you provided was someone who wants to use multiple power supplies to obtain higher capacity for the same load. That is not what you asked for and it doesn't apply in your case.