The three different versions are all based on the same part, and use the same model and texture. They exist to cater for different opinions on the number of crew the part should hold:

Quest Mk1 Probe Core - Probe - It's even smaller than the current Mk1 Command Pod, so it should be too small for kerbals.

Quest Mk1 Command Pod - Crew 1 - It's the equivalent of the current Mk1 Command Pod, so it should also hold one kerbal.

Quest Mk1 Original Three - Crew 3 - The original part held three crew, so this should too.