Without external forces and assuming a binary fission process, the third generation of a bacterium would have approximately 87.5% matching DNA sequences with the original parent cell. Each bacterium shares one complete copy of the DNA with its daughter cells, and while there might be mutations, if we consider only the unchanged sequences, the remaining sequences would be consistent through generations.
However, the options provided do not include 87.5%. Given the standard choices, the nearest correct answer for matching sequences for the third generation could logically be concluded as 100%, if all copies remain unchanged. Bacteria reproduce asexually, so their genetic material would be nearly identical, especially in the absence of mutations.
The most accurate answer based on the options given is 100%, assuming no mutations or variations occurred during the replication process.