I would like to know if it is possible to secure several azure web apps with one SSL Certificate.
Example:
webapp1.azurewebsite.net => app.com
webapp2.azurewebsite.net => app2.com
webapp3.azurewebsite.net => app3.com
3 different domains 1 one SSL Certificate
You can use a wildcard cert, *.foo.com. They are usually more $$ than a non-wildcard cert, however.
Your scenario, won't support wildcards, however because your you are using top-level domains (a.com, b.com etc), if they were a.foo.com and b.foo.com that would work.
Another option, and I don't know which CAs support this, is to use Subject Alternate Names https://en.wikipedia.org/wiki/Subject_Alternative_Name