To answer this question we need to obtain the amino acid chain to understand how each nucleotide changes, affected the amino acid sequence, and see if makes difference or is a silent mutation.
Original DNA strands
1)A G T T T G C A C T T A T A G A G G A T G A A G C C G C A C c T A C A T C A
2)A G T T T G C A C T T A T A G A G G A T G c A G C C G C A C G T A C A T C A
3)A G T T T G C A C T T A T A G A G G A T G A A G C C G C A A G T A C A T C A
Transcription to mRNA
1) A G T T T G C A C T T A T A G A G G A T G A A G C C G C A C C T A C A T C A
U C A A AC G U G A A U A UC U C C U A C UU C G G C G U G G AU G U A G U
2) A G T T T G C A C T T A T A G A G G A T G C A G C C G C A C G T A C A T C A
U C A A AC G U G AA U A U C U C C U A C G U C G G C G U G C A UGU A G U
3) A G T T T G C A C T T A T A G A G G A T G A A G C C G C A A G T A C A T C A
U C A AA C G U G A A U AU C U CC U A C U U C G G C G U U C A U G UA G U
Comparing mRNA strands and identifying mutations
1) U C A A A C G U G A A U A U C U C C U A C U U C G G C G U G G A U G U A G U
2)U C A A A C G U G A A U A U C U C C U A C G U C G G C G U G C A U G U A G U
3)U C A A A C G U G A A U A U C U C C U A C U U C G G C G U U C A U G U A G U
Translating to amino acids
1) U C A- A A C- G U G- A A U- A U C- U C C- U A C -U U C- G G C- G U G- G A U- G U A- G U
Ser - Asn - Val - Asn - Ile - Ser - Tyr - Phe - Gly - Val - Asp - Val
2)U C A- A A C- G U G- A A U- A U C- U C C- U A C- G U C- G G C- G U G- C A U- G U A- G U
Ser - Asn - Val - Asn - Ile - Ser - Tyr - Val - Gly - Val - His - Val
3)U C A- A A C- G U G- A A U- A U C- U C C- U A C- U U C- G G C- G U U- C A U- G U A- G U
Ser - Asn - Val - Asn - Ile - Ser - Tyr - Phe - Gly - Val - His - Val
Compairing sequences
1)Ser - Asn - Val - Asn - Ile - Ser - Tyr - Phe - Gly - Val - Asp - Val
2)Ser- Asn - Val - Asn - Ile - Ser - Tyr - Val - Gly - Val - His - Val
3)Ser- Asn - Val - Asn - Ile - Ser - Tyr - Phe - Gly - Val - His - Val
Step-by-step explanation
We can see that the change is in the first letter of the codon for sequence one and two this lead to a change in the coding amino acid meanwhile the mutation in the third sequence is on the third letter of the codon, so the coding amino acid does not change, this is because the genetic code is degenerate, this means the most important letter are the two first, the third can have higher variability and won't change the amino acid in many cases. Now for sequences 1 and 2, there is a change in amino acid, so this can affect the protein structure, it depends on the protein and where the amino acid is located in the tertiary structure the severity of the impact of the mutation, can happen that has no effect, or it impacts is low, or can be serious and prevent its function. On the other hand, the third sequence has what is called a silent mutation, as the genetic code changes the coding amino acid is the same so the structure is not compromised.