Instead the only option I see are the last 3 addresses that were generated/used by Armory. Is this all I have to choose from?
Every time you send bitcoin a new address is created for receiving the change. If you are looking for a particular address to use for sending bitcoin and you can't find it. that is probably because that address has no more bitcoin in it. Those bitcoins are likely now in one of those last 3 addresses that you are seeing.
If you want to forgo your privacy and always send bitcoins from the same address, you can redirect your change to a particular address and then use coin control to select that address.