the reader can go up to 4.9m. you just need 3m out of it. you have 1.9m to lose.
1) you don't need circular antenna polarized antenna if you know the orientation of your RFID tag. you can align you reader to be on the same direction as your reader if possible.
2) probably you will be fine with the linear polarized antenna as you have 1.9m to lose. your reader is high power (27dBm)
if you looking for the low-cost custom antenna for your application, you can contact me as we are expert in that area. You may face problems due to losses if plug and play antenna impedance doesn't match the Sparkfun board.