Electrical – How to convert this into a down counter

You can create a 3-input, 3-output circuit that maps one state in the count sequence to the next. Then feed the 3 Q outputs of the FF's into this combinatorial circuit, and connect the output of the combinatorial circuit to the 3 D inputs of the FF's. Connect all 3 of the FF's clock lines together, and then whenever an active edge comes along the flip flops act like a 3 bit register, and just load up the next value.

Hmm, you could add a NAND3 gate from the first 3 outputs to the first D flip flop input so you'll get the first '1' in to the counter when the first 3 outputs are all zeroes. Or you could play with the resets. Or make some other combinatorial logic arrangement, depending on what your application for the ring counter is.