* Is it enough one cell with 8/8/8?
In theory yes- you already worked out the site’s capacity correctly. In practice probably not, since this assumes that you have the site in the centre of an ‘ideal’ flat, circular city with uniform traffic density. You are far more likely to find that traffic is concentrated on one or two sectors, and that the topography may mean you need more than one site to provide coverage.
* 5500 x 0.025 = 137.5 Erl, but how can I transform 3 TCH in Erl?
Easiest way is to treat the 3TCH you want to be available as either 3E more traffic or 3 less TCH for circuit switched- this depenmds on whether or not you plan to dedicate the channels for GPRS.
* If it takes 3 TCH off, how much Erl can handle the cell? Maybe I have to
consider 56 lines with a GoS of 3% that is equal to 47.7?
See above.
* What if I take 4 TCH free per sector for GPRS DATA? Still is it possible
to be complied?
See above.