I am going to start three sites this week for adsense earnings.I want to get some US related niche which can pay high and which can be marketed in US./
I think Celebrity , Properties and Banking sector is mainly focused over there . From last few weeks Stock market and Forex also booming .
I'd suggest stop thinking "make money fast". MfA can cost you your AdSense account. There is pretty much no way you are educated enough on three different and random niche topics to start a site that would be of value to your visitors (do you actually know the definition of a Niche?) Here are some pointers: http://www.onyxbits.de/node/57
That sentence made pretty much no sense at all. A niche is a segment of a market, that is not mainstream (see wikipedia for a definition). Meaning, there is no "the US niche", there are simply a lot of topics, some more crowded then others. If you do not even know, what topic you want to serve, then all you will produce is "Made for AdSense": Low quality content with the sole purpose of showing ads (as you pretty much stated in the OP). Also, I would like to remark, that if you had any idea of what a niche was, you'd realize, that a niche by definition does not attract a lot of traffic and has only few suppliers of products, who do not even have to bid high on their keywords, because there is a lack of competition.
Get started with either Healthcare, Finance or Insurance. Those are typically the higher paying niches here in the US. Competition might be fierce so I'd recommend going after longtail keywords.