3-Terminal Capacitor: A capacitor having a third terminal which connects to ground. A three terminal capacitor may be a through-hole with 3 leads or surface mount with 3 pads. Primarily 3-Terminal Capacitors are multilayer ceramic chip capacitors which are used in EMC / RFI Suppression conditions.
The structure of a 3-Terminal Capacitor appears almost as a Feedthrough Capacitor which is shown to the right.
A feed-through capacitor can be considered to be a kind of a 3-terminal capacitor. The difference between a Feedthrough Capacitor and a 3-Terminal Capacitor is the additional inductance in the ground lead [not shown]. However the two inductors that are shown represent lead inductance and not true inductors.
Surface Mount 3-Terminal Capacitor
3-Terminal Capacitors have a smaller equivalent Series Inductance [ESL] because the distance from a positive electrode to a ground (GND) electrode is shorter and the cross-sectional area of a rectangular solid is larger. In order to make ESL smaller, capacitors have to be shorter in length and longer in width. This principle ultimately leads to a three-terminal shape, in which the power line is incorporated in the component, and the distance from the power line as a positive electrode to the GND electrode is shortest.
3-Terminal SMD Capacitor
Through-hole 3-Terminal Capacitor
There are a number of possible capacitor styles that could be 3 terminal devices, these are just a few examples. It appears that in general 3-Terminal Capacitors use a ceramic dielectric [from what I've found].