Zenit-3 replaced the original Zenit and Zenit-S in the early 60's (production from 1960 to 1962). The mechanics were similar to the Zenit-S (mirror was lowered with a [famous] string), but a new great feature was introduced - advance lever replaced a winding knob of the early Leica copies. The camera is still loading from the bottom.The camera has a flash synch mechanism with the adjustable synch timing advance (from 0 to 25 ms). Also, the shutter speed can be selected before of after the film is wound and the shutter is cocked.
The camera shown here is modified, the rewind knob being non-original. The lens is also of late 1970's manufacture.
The Zenit-3 shows outstanding workmanship, the camera is very solid and was apparently expensive to make. This explains why it was in production for only two years. As any early Zenit it is quite uncommon now (source: www.fedka.com).