You can definitely run 3 websites in the same instance. Check out this MSDN article that shows you how to form your configuration file such that you can host multiple websites within a single role. One thing to note though since you mentioned "scaling on demand" - when you scale an instance with multiple websites, you are scaling the instance, which means all of the sites will scale together. You can't scale just one of the sites on the shared instance.
For the databases, in theory this can be done, but it would be "manual" in that you would have to all of your tables across the three databsaes in the same database and you would probably want to prefix them with some sort of indicator so that you know which table belongs to which application. This is certainly not a recommended practice, but if it works for your solution, then there is nothing technical preventing you from doing it. If at all possible, I would recommend multiple databases.
