I can speak to the USP and P2000, but will leave the P30 commentary to people who actually own one.
Differences/Benefits:
-Grip. The P2000 has interchangeable backstraps, making it comfortable for a wide variety of shooters. The grip on the USP is too large for some, and the stipling can also be uncomfortable under prolonged shooting.
-DAO, DA/SA, DA, or SA? Between these two models, you can have any style you want. The most common version of the USP is the V1, which can be a single action with a sweep-down safety if you prefer. Or, sweep the lever to the bottom to decock. At that point it's your option whether you prefer DA/SA carry with or without a manual safety. Other versions of the USP are available, having the lever act as a decocker only for example. On the P2000, you have two versions: One is equipped with the LEM trigger, making each trigger pull the same, and the other is DA/SA with a decocker located at the back of the slide.
-Rail. The USP is equipped with the proprietary HK rail system, whereas the P2000 has a standard accessory mounting rail.
-Action. The actions of the pistols are similar, both Browning style "tip-up" barrel.
The P2000 is also less "blocky" than the USP, and smaller than the full-size pistols. A more ballpark comparison would be with the USP Compact series, which are similar in size the P2000 but have the manual safety of the USP. Magazines are interchangeable between the USPc and P2000 models of the same caliber.
Why does H&K make so many of what appears to be the same basic type of weapon?
Very shortly you'll have a line of posters ready to tackle this question with an overused line about sucking and hating.