Help making a buffer solution?

You are exactly correct. Formic acid would be the best as its pKa is closest to the pH desired. Note in th HH equation that when the ratio of the salt to acid is 1, the pH equals the pKa, so if you use 0.1 M of each (acid and conjugate base), the ratio will be 1, and the pH should be 3.74. Hope this helps.