all three Dna samples have different charges. All DNA samples have the same (negative) charge because nucleic acids have a phosphate backbone. This negative charge is the reason that DNA migrates toward the positive end of the gel

Correct, DNA is a negatively charged nucleic acid. This is because the phosphates present in the sugar-phosphate backbone are negative and hence, make the DNA acidic. That is why DNA is called deoxyribonucleic acid

In tests like gel electrophoresis, the DNA will move towards the positive end of the gel. This is because positive attracts the negative. A DNA sample can be separated on the basis of the negativity it possesses.