Check out this article on batting average by position.
Cricket Records, Information and Everything Cricket: Progression of Test Batting Averages of Various Positions
No real surprises here! But a few interesting points -
1. Difference between the top and the bottom batting positions seem to be increasing over the years. Specialization?
2. West Indies and Australia seem to have had historically the best No. 3 batsmen. A quick search for WI reveals players like Headley, Lara, Weekes, Richards etc. Even Sobers made his 365* while batting at 3. For Australia, of course Don Bradman himself may have single-handedly brought up his country's average, not to mention others like Chappell, Ponting etc.
3. This is most surprising to me. Batting Averages from 1925-1950 hold their own, even against the modern generation. Weak bowling, LBW laws?