A three letter Callsign is pretty hard for the average listener to detect. Very frequently it's indistinuishable from the "shortened" call sign other stations may use with it's branding. Dropping the K or W is not part of a legal ID but is still commonplace. WHCN calls itself HCN... etc.
But at one time it was the 4-letter calls that were uncommon. The flood of broadcasting service authorizations that began in December of 1921 overloaded the "recycling" three-letter calls. Before the FRC hit that wall they managed to assign three-letter callsigns to about 200 broadcasters.
There are less than 70 remaining in use today. These are virtually all heritage calls. the list includes KYW-AM Philladelphia, KUT Ausin, WRR Dallas, WSM-AM& FM Nashville, KOA Denver and others.
They've been dwindling. As Barry Mishkind explained "As of January 1, 1932 there were 93 three-letter calls, all on the AM band (FM and TV stations didn't exist yet). Over the next 74 years the number of three-letter calls have declined by 36 on the AM band, from 93 to 57 However, of the 36 calls which have disappeared from the AM band, 12 still live on as FM or TV stations so counting all bands the current number of surviving unique three-letter calls is now 69 . " What he's pointing out there is the preservation of heritage calls in markets by parking them on FM and TV sticks. Was the heritage branding was reused to help the transition of viewers & listerners over to the new mediums? perhaps.
He continues "Since 1932, 24 calls have completely disappeared from the airwaves. Three calls disappeared due to station deletions--WNJ, WOQ and WOS, while the other 21, are gone due to call changes and station consolidations: WFI, KTM, KVL, WPG, WLB, KLS, KPO, KQW, KLX, WOV, KTW, KOL, KMO, KXA, KRE, WHN, KWK, KSO, KOH, KYA and WOW"
The interesting movement lately has been the FCC's tolerance for the movement and reassigmnet of old 3-character calls to the AM band. On 07/01/2005 WCMS in Norfolk returned the WGH calls to the market. WRTH St. Louis became WIL in 06/29/2005 and in 03/15/2000 Los Angeles KKHJ became KLA.