Plain English Meaning
An altimeter with three hands on its face, each one showing a different scale of altitude. You read all three together to get your full altitude, similar to reading hours, minutes, and seconds on a clock.
Full Definition
A sensitive altimeter that displays altitude using three pointers on a single dial: a long pointer indicating hundreds of feet, a shorter pointer indicating thousands of feet, and a small narrow pointer indicating tens of thousands of feet. The instrument is driven by an aneroid mechanism that responds to changes in static air pressure, with a Kollsman window allowing the pilot to set the local altimeter setting.
Context Anchor
Seen on older or training aircraft instrument panels and in FAA instrument explaining how altimeters .