Ideally, I like to have at least 2 plus defensive SS up the middle, one at SS and one at 2B. I look for 90/90/90/90 if at all possible. In CF, a third 90/90/90/90 would be awesome, but I'll sometimes go less on the glove/arm, but I keep the 90 range in CF if I can. 3B is generally what those who don't care about defense would consider an acceptable SS for them - below the SS recs in one or two categories, but not by much.
The fact of the matter is, if you focus that much on D at those 3/4 spots, you're not likely to get much offense out of them unless you are very fortunate. I try to compensate somewhat by treating 1B/LF/RF as relatively unimportant defensively. For example, I haven't much problem with playing a guy who is recommended as a LF in RF, or a guy recommended as a 1B in LF. I won't go so far as to play a true DH anywhere (even 1B) but other than that, I tend to cut corners at 1B/LF/RF if I have to.