Final answer:
The third generation of a bacterium and the original parent cell will have approximately 100% matching DNA sequences without external forces, barring any spontaneous mutations.
Step-by-step explanation:
The question asks about the matching DNA sequences between the third generation of a bacterium and the original parent cell in the absence of external forces. In such a case, the third generation would still have 100% matching DNA sequences with the original parent, minus any changes due to spontaneous mutations which are typically rare. This is because bacteria reproduce asexually through binary fission, which is an exact duplication process. Therefore, barring mutations, the descendants have identical DNA to the parent bacterium.