To raise salt by 3000 in a 27' by 4' deep round pool will take about 10 1/2 40 lb bags of salt. However, you can't always count on the water starting with zero salt or the calculation being exactly right, so I would start with 8 bags and then test the salt level to see where you are and adjust from there.
You can use my Pool Calculator, see the link in my signature, to make this kind of calculation.