[1a] I have three addresses that I need to use for letters - as far as i can see, I think I can only add two sets of user contact details (Home & Business) - is that correct?
[1b] Alternatively, could I use the database for my third address? That would mean using the database for two sets of details which would both be manually chosen - is that possible?
As only one database at a time can be connected to a document, it won't work to use it for two simultaneous selections which are independent from each other.
Of course you could simply create three templates, each one containing one of your addresses as fixed text, and choose one of them when you start writing a letter.
If you want to do it with one template, you could use calculations with if-conditions. Just have a look at the attached sample file. If you insert a "2" or "3" into the yellow field, the dates change accordingly. The yellow text field is only visible on screen and will not be printed. (Instead of a text field, one could also use a dropdown list, but I understood you prefer keyboard to mouse.)