No matter where your mother's brother ranks, you have to call him uncle. Your mother's third sister-in-law is also your third uncle's lover, so you have to call him third aunt. If it is your father's side, and your father ranks fifth, then you should call your father's third brother and third uncle, and your lover (your mother can also call her third sister-in-law) should be Sanauntie, or commonly known as "Sanma".