If the threepence is plated (and shows signs of what lies underneath) then it's probably been plated by someone after it left the mint, for a laugh. But if it is actually a different metal all the way through, then it's probably a mint error. Threepences in Nickel are not unheard of and actually there was a Canadian coin which looked very similar to the UK 3d but was made of nickel. The Royal Mint have been known to rarely use foreign blanks. Are there signs of yellow brass coming through, or signs of chiping of potential plating?