ThreadRipper is using TR socket, which is much wider in size and has more connection points therefore ThreadRipper CPU don't and won't fit into Ryzen socket (AM4).
If you're asking if ThreadRipper 3 will fit into TR4 socket of older motherboards (bought for ThreadRipper 1 or 2) - it will.
qbert, the only Ryzen 3 platforms available, that I know of, today are for AM4 sockets. Since no TR3 processors has been announced, that I know of, then we can only assume they will continue to use the TR4 socket. If these assumptions are accurate, then the answer to your question is no. Enjoy, John.