Can I add a multisignature address to bitcoin core? Can I then add the private keys (assuming I have them of course) and do a spend using that wallet?
To add a 2-of-3 MustiSig address to bitcoin core you will need to execute in client console the following command (in your example):
addmultisigaddress 2 '["03d6a0edc630c29f649500d592091039d7f3c942ec07bb7ebdabfe5727c58964fd","0305143caa81bc0fec9237f594939d0ceb667cb78e451264e8d2a677648a871aeb","0306c73d2aea0a492e268dd1b3ed1ba1bbdc54c1d4a3a46d9d74fe81f3e6a301af"]'
Result:
3NUEUQspbf9LK3mRN13pLNyzeEK4Ep1HRC
In order to spend the funds from the MultiSig address you will need to import 2 of 3 private keys that correspond to these public keys:
importprivkey KwyKy7C7f8pHw1GizvWhLpE6GB13ZcfNJoX98MHiwwz1x77MksQq
importprivkey Kyw4KDi8Lre2WhpTJt8p8gfTEkpSfqVv8dv3bJ75cmuyvFHQ7HBy