Why only one station in the Bay Area with three call letters?
In the early/mid 20s, 3-letter call signs were the norm. It wasn't until 1924 that a Bay Area station got the first
4-letter call sign in Northern California: KFRC. But 3-letter call signs were still issued (e. g., KYA in 1926). And as w9wi said, the stations who still have the 3-letter call signs got them while the Commerce Dept. still issued them and they are grandfathered as the FCC doesn't and hasn't issued 3-letter call signs for decades.
For KNBR's first 25 years, its call letters were KPO, KDIA was KLS, today's KNEW began life as KLX, w9wi already mentioned KCBS...KFBK used to be KVQ, etc. I didn't know that the Federal Radio Commission issued 3-letter signs until 1930, though. I didn't know when the "stop point" was for the 3-letter signs.
I believe there was a radio station in the Bay Area a long time ago with the call letters KROW
KROW (960 AM) became KABL in 1959 when Gordon McLendon bought it and made it a "Beautiful Music" station. It's now KQKE I believe.
Do call letters stand for anything and who picks the letters?
The FCC gives a licensee three choices for call signs, provided they aren't already taken, of course. Otherwise, I think they just issue an arbitrary set of letters. As for the letters standing for something, the story goes that KHJ stood for "Kindness Happiness and Joy," though what w9wi said about the meaning of the letters being ascribed AFTER they were issued is entirely believable. BossRadioDJ can tell you what KFRC (and others) supposedly stood for after he's done picking brain chunks up off the floor (inside joke, sorry ;D). The "R" and "C" stood for "Radio Clearness." How 'bout it, Dave? A few other examples are:
WLS --
World's
Largest
Store (Sears owned it)
WGN --
World's
Greatest
Newspaper (Chicago Tribune)
Examples of "vanity" call letters are:
KROY --
Royal Miller (original owner/licensee, 1937)
KBEE -- Modesto
Bee (both AM & FM) (McClatchy Newspapers?)
KXOA
KXOB
KXOC -- All sister stations in Northern California (KXOA became KIID Radio Disney, KXOB became KJOY, KXOC became KPAY)
KCRA -- Was supposed to be "KRCA," but an FCC typist made a mistake and flipped the two middle call letters around...anyway, KRCA was chosen because of its NBC affiliation
KTOB -- K
Top
Of
Bay
KVON -- K
Voice
Of
Napa
KECC -- K
Eastern
Contra
Costa (became KKIS) (thanks BossRadioDJ)
KNBA --
Kovers
North
Bay
Area
KSTN --
KSTockto
N
KTKZ -- K-
Tal
k(
Z) (K-Talks)
KVFS -- K
Vacaville
Fairfield
Suisun (became KUIC)
and on and on and on...