The answer is going to vary widely on your requirements, which since you're new to the game, you may not be entirely aware of.
For Rails (mounting machines inside the rack): Square Holes or Circle Holes
To the best of my knowledge, square holes are pretty much the standard everywhere. I can't think of any single server we've had that shipped rails with circular holes. Still, this is a possible consideration that you'll need to evaluate when you decide on servers.
Height Needed for Devices in the Rack
A "U" is a "unit" of measurement for racks. Servers come commonly in 1U, 2U, 3U, 4U sizes, and larger things like high capacity storage, high end switches, blade servers, etc. can be taller than 4U. Figure out how many U's your 3 servers use, consider expansion if necessary (and other appliances like switches, kvm's, and the like), and you have your rack size.
We have a handful of 42U racks (which is more or less the common full size rack height), a single half-rack (24U), and a rack on wheels mobile (sorta) unit which is either 8U or 10U.
A Personal Pitfall
One of my first jobs where we had a formal datacenter, we bought a solid door rack. I believe it was some kind of plastic on the front and back doors (the side walls are always solid, but removable). The only holes were in the bottom of the rack, and the top. IIRC we had raised floor ventilation, the theory being that hot air rises and exits the rack. Don't bother with these racks, we lost a couple of servers, and nearly burned our hands trying to move other ones. I'm a huge advocate of front to back airflow (assuming your airflow is adequate).
I'm foregoing my recommendation because they don't make small racks, and if you're looking for only 3 servers (+1U for a switch?) this company doesn't make racks shorter than 24U to the best of my knowledge.