USB-C accessories and cables will work with a Thunderbolt 3 port. All Thunderbolt 3 ports are also USB-C ports, using the same style connector. I don't have any experience connecting to amplified speakers but I'm guessing that what you'd need would be a DAC with USB input to take the signal from the Macbook, convert it to analog and then on to the active speakers using a standard pair of analog cables, either RCA or AES, depending on what the DAC and active speakers have.
But, as I said, I don't have any direct experience with this so please confirm elsewhere.