This is the 3 element yagi style beam antenna. This is a fairly generic antenna design, which was offered in slightly different variations by many different companies. The most basic version of this design sported a direct coaxial feed, a 6 foot boom, and about an 8dbi advertised gain. Newer designs employed gamma matching, a longer boom and over 9dbi gain.
This antenna is a directional design, which means that it concentrates its receive and transmit signals in one major direction only. This necessitates using a rotator device so that the operator can point the antenna in the direction in which he wants to communicate. The 3 element configuration is the smallest yagi style antenna typically found in the CB service. The 3 element antenna is fairly small in both size and weight, and can be handled by a fairly light duty television quality rotator, and can be held by standard duty steel mast. In short, it does not require a lot of special considerations over a standard omni-directional ground plane antenna, which made it a popular choice for someone who wanted the performance of a beam, but did not wish to deal with the hassles associated with larger beams.
Besides the signal improvement that can be had (in one direction at a time) from a beam antenna over an omni, the other big feature is front-to-back and front-to-side rejection. While the beam concentrates more signal in a single direction, it also greatly reduces signal in all other directions, the deepest null being about 120 degrees from either side of the front. Some 3 element designs can approach 35+ db of front to side rejection, which can be a real asset if you live in an area with many CB operators who operate on other channels, and reduction of bleed over is necessary to your continued hobby pleasure. I had a few different 3 element beams and I could see first hand the advantages of this signal rejection. One of my CB associates lived barely 8 doors away from me. On an omni antenna, he would give me a +30 db over "S9" signal. With the beam pointed carefully away from him (The deepest null is very sharp), I could drop his signal down to just above S9, which was about equal to the signals from stations over 3 miles away! At that signal, he did not bleed me, nor could he override as many people. He actually gave me a stronger signal when I disconnected the coax cable from the back of the radio. Truly amazing.
In my local area, a bunch of people ran 3 element beams at some point, including myself, Steve, Dennis The Menace, Tigershark, Jimmy, Joe, and others.