If you're just interested in casual play then I would say, no you do not have to by 3 or 4 of everything. The only exception to that would be for instance if you wanted to make a pure Mi-Go (creature/character subtype) themed deck you might have to track down multiples of certain releases.
Start off with-
1st Core Set
1 Secrets of Arkham
Then if you gotta have more-
2nd Core Set for more consistency
*The Order of the Silver Twilight
*The Rituals of the Order
The last 4 I suggest to include the added 8th faction Silver Twilight but you could certainly skip them if you feel there is no need. The Yuggoth Contract or Forgotten Lore Asylum Pack series would be a good alternative. If you develop an affinity for a particular faction you could consider eventually picking up one of the faction specific deluxe box sets as well.
Cardgamedb.com (just purchased by FFG) has card spoilers so don't take my word for it! Check out the card images to decide for yourself.
Edited by Hybrid, 27 August 2013 - 03:39 PM.