The Voskhod capsule was very cramped, and the three-cosmonaut flight was essentially a propaganda stunt. From Wikipedia:
[Voskhod 1 was] the first flight without the use of spacesuits...
The three spacesuits for the Voskhod 1 cosmonauts were omitted; there was neither the room nor the payload capacity for the Voskhod to carry them. ... The only other space flight in the short Voskhod program, Voskhod 2, carried two suited cosmonauts — of necessity, because it was the flight on which Alexei Leonov made the world's first walk in space.
There was enough room for three people, or two people wearing bulky pressure suits, but not both. I believe both crew members of Voskhod 2 wore full pressure suits. The misson used an inflatable, external airlock -- not the kind of thing the non-spacewalking crew member would want to trust his life to.