There are 3 different Series 3 models.
The original was the TCD648250, with 2 cable card slots on the back panel.
It needs a separate cable card for each of its two tuners, either or both of which can be either an S card or an M card, but because the M card specs weren't finalized in time it can't use just one M card.
Apparently TiVo put the letters HD on the cardboard box for that model.
Then they introduced the TCD652160 and called that the HD, creating all sorts of confusion.
They also introduced what's basically the same machine with a bigger hard drive and THX certification and called that the HD XL.
On those two models the lower part of the front panel swings down and provides access to the cable card slots.
You can use one M card and it'll take care of both tuners, or you can use an S card in each slot to take care of both tuners.
I don't know what happens if you put in 2 M cards.