checking traffic of any site?

Advert test
#2
yes. it is possible but site should be your own and analytics code should be implement into all pages before ending </body> tag.
 
#3
Yes,
Google Shows impressions and hits for a particular keyword in Google analytics as well as in your Google webmasters account.It also shows organic traffic as well as traffic from online advertising such as links and Google Adwords traffic separately.
 
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