I believe the threepenny in threepenny bit was pronounced thrupnee
There is often a slight hint, or more than a slight hint, of the middle syllable, as also in haipnee for halfpenny, often written ha'penny. For example in the usual tune for the children's nursery rhyme "Christmas is coming, the geese are gettting fat" there are 3 syllables provided for ha'penny. Also note the pronuciation tupnee (with again a hint of a middle syllable) for twopenny, often heard in the combination twopenny-ha'penny, used to mean insignificant, of little value, or, in the case of a person, of little influence, or jumped-up. Plurals are ha'pence, tuppence, thruppence.
As a child, my mother often used to call me "a daft hapeth", as it sounded to me, when I did something silly. It was a long time before I worked out that this was a ha'p'orth, a halfpennyworth.
Edited by: Sir Isaac Newton, master of the Royal Mint