My iPhone 3gs sat unused for a couple years. I used a iPhone 4, then 4s. While I like the 4s, I've activated an unused number to have my 3gs to use again. So, I do not recommend you sell or give away your 3gs if it is still working. To me, the shape and therefore, the feel of the 3 series iPhone is what I would want in a new iPhone 6.
Apple....are you listening........?