What don't you like about the first two? Or it's weekneses. Just curious.
The first two are their verson of the bluesy hybrid that most bands were doing at the time. Hedrix had already been the man at that. III is when they added some folk influence and expanded their range, which was a sign of things to come.
The Beatles did the same thing with Rubber Soul, which was also the turning point before their best albums. The Stones did it with Aftermath.
The heavy hitting bands wanted to be taken seriously, like Bob Dylan, and their music got better when they added that folk influence.