I have tried to look up those three at
http://www.realhardtechx.com/index_archivos/Page1530.htm to find out more about them. The Zalman is made by an OEM that is utterly unknown to me. Otherwise, I don't recall any Zalman unit getting high praise from anyone, ever. No reviews for it were listed, making this unit an unknown.
You don't list the specific EVGA model, but if it is cheap, it is probably the HEC-built 500B. Though not a great unit, it passed testing in the few reviews that are listed; I'd call it no less than "safe."
The Enermax is built by CWT, which has a mixed reputation. There is only one review showing, from a site I don't recognize, but it appears to do some of the "necessary" testing including voltage regulation and ripple, and the unit did well. The problem for me is that the similar 600W version, also CWT, did very poorly in a HardwareInsights review. The two units may or may not use the same platform, but probably do. It could well be the same platform used by the older Corsair CX (not the new one), which also had somewhat erratic quality depending on the wattage; none of them "sucked," but some didn't last, due to inferior capacitors used.
In your case, of those three, I'd probably choose the EVGA 500B. While not a "great" unit, it isn't bad (per reviews), and your system isn't going to be stressing it much at all (likely 200W-220W running a heavy load; 150W more typical). My second choice would be the Enermax. The 500W version appears to at least be decent, and as I mentioned you won't be stressing it.