Greetings,
All 3 have something that shines uniquely for each of them. In some cases some of those attributes can be combined. For starters, see which one fits your hand best. Then it comes down to trigger and features.
The Glock has its own unique trigger operation using their "safe action", basically a relatively light double action pull with a short reset. Some folks don't like how the Glock sits in the hand though. I was one of these folks and had a Glock 22 with the finger grooves ( which I can't stand) and eventually sold it. Now I have a 2nd gen (without the finger grooves) Glock 19 and absolutely love the gun. I find the Glock trigger works really well for fast combat/action type shooting, but find it tough to squeeze tiny groups out of it. I also think that if I shot this gun and this gun alone for awhile, I would start to realize it's accuracy potential. Plus there are a ton of G17 used mags out there that aren't that expensive for pre-bans.
The Sig has one of the best DA/SA combos going, and is also a very accurate handgun as well. Ergonomics are well thought out and the gun just shoots so well it's boring. If this gun is just going to be a range shooter to get tiny groups, for me the Sig 226 and the USP shine here. I have a mid 90's one with the folded sheet metal slide and it's one of my favorites. If you look at CDNN, SOG, or KY Imports, I believe they examples of these pistols as used police trade ins for under $400.00. These places also have used hi-caps for around $40.00. By the way, this is my wifes bedside gun.
The USP is one of my favorites. I have the USP45F and it is my bedside gun. Would very very much like to get the 9mm variant. This gun has it all. It has outstanding accuracy, like the Sig, but also has cocked and locked capability. The mag release is an issue for some folks, personally, I find it is my favorite release style. The USP 9mm was designed from the USP40, so the 9mm is built like a tank....extremely robust. Also, HK has their LEM trigger out. The LEM trigger has a Glock type feel to it, in that it's a very light double action type trigger with a short reset. It can be had in weights as low as 4.5 lbs. Pre ban mags for this gun are usually more expensive than the Sig or Glock.
So, in answer to your question, I would take the USP9 with the 4.5lb LEM trigger.
Hope this helps.
Ken