PL actually stands for phone line since that connector got it's start in telephony (I was with the phone company for about 12 yrs). I actually referred to the PL plugs as TS/TRS, but it usually confused a lot of people so I reverted.
The RCA or phono plug, since it was regularly used with turntables (or phonographs), got it's name from it's maker, the Radio Corporation of America or more commonly known as RCA.
From wiki: The XLR connector is an electrical connector design. XLR plugs and sockets are used mostly in professional audio and video electronics cabling applications, for microphones and line level signals. Home audio and video electronics normally use RCA connectors for line level signals generated by a preamplifier. Phone plugs are also used for microphones in home and computer applications.
In reference to its original manufacturer, James H. Cannon, founder of Cannon Electric in Los Angeles, California (now part of ITT Corporation), the connector is colloquially known as a cannon plug or cannon connector. Originally the "Cannon X" series, subsequent versions added a Latch ("Cannon XL") and then a Rubber compound surrounding the contacts, which led to the abbreviation XLR.[1] Many companies now make XLRs. The initials "XLR" have nothing to do with the pinout of the connector. XLR connectors can have other numbers of pins besides three.