The choice of handgun depends on what you shoot well. I have a STI double-stack 1911 in .40S&W, but others may prefer Glocks. As the pistol is a comparatively minor part of most 3-gun matches I've shot, just use whatever you have confidence in. Hi capacity helps a lot.
Caliber choice depends on the rules you are shooting under, IPSC vs IMGA. If IPSC - or if you will shoot a lot of pistol matches - then you should consider a major caliber pistol (.40, 10mm or .45) as they score higher. If you are shooting IMGA rules (which govern most matches these days), then there is no power factor system, and a 9mm will give you more shots with less recoil. If you are going to shoot "Heavy Metal" class, then check your local rules - most require a .45 pistol, but some mandate single-stack 1911 only in this class.
Lastly, on holsters, I would err on the side of retention and robustness rather than pure speed.
Hope this helps.