You might also try Trulia.com. I find it a little more up-to-date than Zillow.
Finding the 3 exact models is a little odd to me, since so much of a home's value is in its location. I would expect radically different price points for a house on a 4-lane road vs. one on a quiet cul-de-sac, even if the same model and in the same neighborhood. However, if you're in the area, a walk around the neighborhood should give you an idea as to which homes are same model as the one you need; if you note the addresses down you can then look them up in the property tax database for your county.
Of course, remodels, improvements, etc. are also going to change the value of a home, and you usually can't see that from the outside.
"You'd be surprised at how much it costs to look this cheap." -- Dolly Parton