“3U” refers to the height of a module or rack format, with “U” being short for “rack unit.” A rack unit is a standard unit of measure for the height of equipment that is designed to be mounted in a 19-inch wide rack or a 23-inch wide rack. One rack unit is equal to 1.75 inches (44.45 mm) in height.
In modular synthesizers, the 3U format means the module or rack is 5.25 inches (133.35 mm) tall, as it is three times the height of a single rack unit. The 3U size is commonly associated with the Eurorack format, which is a widely adopted standard for modular synthesizers. Eurorack modules are typically 3U in height and have a horizontal width measured in “HP” (horizontal pitch), where 1 HP is equal to 0.2 inches (5.08 mm).
The 3U format has become a popular standard for modular synthesizer systems due to its compact size, allowing for a dense arrangement of modules within a given space, making it more portable and versatile.