Neeraj,
Actually, if you have 3 TRX on a same sector (2 with 8TCH and 1 with 6TCH, as you mentioned) you’ll have 22 TCH wich gives you 14,9 erl (at 2%).
And the calculation has others considerations like percentage of Half-Rate traffic, number and type of signalling channels, dedicated GPRS channels.
But just as you explained, all we have to do is sum all TCH avaiable and look up the Erlang B table.
Regards