T3

T3 is the type of data framing used on a DS3 circuit. T3's do not exist as a physical thing. Granted when you hear someone say T3, they are usually refering to a DS3 line, but they are (probably unwittingly) calling it by the name of the encoding standard that runs on a DS3 line.

Same goes for the universally misnamed DS1 circuit -- T1 encoding but DS1 line.

T3 is also another term for triiodothyronine, a thyroid hormone which helps to regulate the metabolism.