The first result, after one generation, showed that the DNA had an intermediate density rather than heavy or light. This ruled out conservative replication, as, in that case, the result wouldn't have shown intermediate density, but rather equal amounts of heavy and light DNA. On the other hand, intermediate density would be possible with both semiconservative and dispersive replication.
The second result, after two generations, showed that one part of the DNA had intermediate density and the other part had light density. This ruled out dispersive replication as in that case the DNA distribution would have been same between the strands and the resulting density would have been lower than the intermediate one.
The third result, after several more generations, showed that now a larger portion of the DNA had light density, synthesized from the first and second generations, and a smaller portion of DNA had intermediate density, sythesized from the heavy and light DNA. This proved the semiconservative hypothesis and was a major step in developmental biology research.