I don't think the distance from Orissa coast to the Bay of Bengal is 3000 km. It's more like 1000 km. But the apogee is abnormally high 900 km! 3 times the normal height. So this launch was done mostly to test accuracy.
The missile itself is obviously much more advanced than the original Agni-2. I doubt the original Agni-2 was ever even reliable, but the Agni-4 at least is designed to be reliable. I think DRDO figured out that Agni-2 and Agni-3 had fatal design flaws, so they redesigned it incorporating carbon fiberglass casing technology -- Agni-4 and Agni-5 is the result.
India is roughly where China was 1970 to 1985. 1970 was the first successful test of the DF-3. 1985 was the first successful test of the DF-3A.
Boy!Agni 3 is in a different league altogether.There is no designe flaw in it.