Designing the molar ratio for Gibson Assembly involves optimizing the concentrations of the vector and inserts to ensure efficient assembly without excess or insufficient DNA fragments. Here's a step-by-step approach to determine an optimized molar ratio:

1. **Calculate Molar Amounts**: First, convert the DNA amounts to molar amounts using their respective lengths and molar masses. The molar mass of double-stranded DNA can be approximated using the formula:

\[ \text{Molar mass} = \text{bp length} \times 660 \, \text{g/mol/bp} \]

Given:

- Vector: 13 kb

- Insert A: 1.7 kb

- Insert B: 4.2 kb

Calculate the molar amounts of each DNA fragment.

2. **Determine the Initial Molar Ratio**: Initially, you proposed a 1:3:3 molar ratio (vector:A:B). This ratio could work, but it might be suboptimal depending on various factors such as fragment concentrations and their efficiency of assembly.

3. **Considerations for Optimization**:

- Vector concentration: Since the vector is the backbone of your construct, you might want to ensure it's present in sufficient amounts to reduce the likelihood of empty vectors.

- Insert concentrations: Inserts should be present in excess to increase the likelihood of successful incorporation into the vector.

- Efficiency of assembly: The efficiency of Gibson Assembly might vary depending on the specific DNA fragments, their lengths, and the enzyme mix used. You may need to adjust the ratios based on empirical data from your lab or literature.

4. **Empirical Testing**: If you're uncertain about the optimal ratio, you can perform empirical testing by trying different ratios and assessing the efficiency of assembly through methods like gel electrophoresis or sequencing.

5. **Optimization**: Based on the results of empirical testing, refine your molar ratios to achieve the desired assembly efficiency. You may need to iterate this process until you find the optimal ratio for your specific experiment.

Regarding your proposed 1:7:3 molar ratio (vector:A:B), it's worth testing empirically to see if it improves assembly efficiency compared to the initial ratio. It's within the realm of possibility, but empirical validation is crucial.

In summary, while a 1:3:3 molar ratio could work, empirical testing is necessary to optimize the ratio for your specific Gibson Assembly reaction. Consider factors such as vector backbone stability, insert concentrations, and efficiency of assembly to determine the most suitable ratio.