According to my big Oxford threepence, threepenny bit (and thrupenny bit in Collins). I don't think it was ever three pence, except perhaps on the coin itself. I can remember feeling rich because my great Aunt gave us twopence (tuppence) a week pocket money. Both always pronounced as one word or said as two/three pennies. Pence or pee was post decimalisation. I can still do all the conversions!