I'm a bit confused,

lets say thier rate for one of the ads is $3.00/1000 imperesions.

where imperesion is 12 hour unique visitor impresion.

and say i have selected 100 different ads to show on my site.

if one of my visitor goes on 100 pages, will fastclick show the user one impresion of eachh ad ? and would that count as 1 impresion for each one of my selected 100 ads?

The ads will be shown on each page that has the code. If they only count one unique per 12 hours, then I guess you don't get paid for that if one visitor goes to each of your 100 pages :)

but they have ads from 100's of different advertisers, wouldn't they show my unique visitor a ad from a different advertiser on each page the visitor goes to?

Yes, they will show, just not be counted as unique. I am not sure we have a fastclick rep here, but I can summons one from another forum to join :)