I am always "teching" my 3 drops depending on what I see the most.
If I face too much Zoo, Hunter and Shaman - I run 2 MC Techs.
If I face more mid-range decks - I run 2 Earthen Ring Farseers.
If I face too many weapon classes, I take Farseer out and put in an Acidic Swamp Ooze.
If I face more Control decks, I take MC Tech and Farseer out and put in a 2nd Alchemist and Cairne.
I've been watching Zetalot play for a long time and if there is one thing I have learned from him is that you have to adapt your deck to the meta.