In the absence of external forces, bacteria reproduce asexually through binary fission, which results in two daughter cells that are clones of the parent cell. Each generation retains nearly all of the DNA from the previous generation.
After one generation, the first daughter cell has 100% matching DNA with the parent. After a second generation, the second daughter cell also has 100% matching DNA with the first daughter cell, and so on.
However, to find the matching DNA sequences between the original parent cell and the third generation (which is technically the second daughter cell), we focus on the fact that all descendants have the same genetic material unless mutations occur, which is generally minimal in bacteria unless specified otherwise.
Thus, the third generation bacterium will have approximately 100% matching DNA sequences with the original parent cell.
So the answer is: 100%.