He said your mobo has 6 RAM slots. You said you had 3GB in the form of three 1GB sticks right now.
You plan to double your RAM to 6GB by adding a fourth 1GB stick and then one 2GB stick, leaving you with 4x 1GB, 1x 2GB and one empty slot. That will work but you won't get proper triple-channel support.
He said you're far better off by simply adding three more 1GB sticks so you have six times 1GB, filling all slots and allowing the system to support triple-channel over your entire RAM. (He said 'add 2 more 1GB sticks', but he either meant on top of already having 4x 1GB or he was confused and meant 'add 3x 1GB' instead.)