Method set up for a chemical separation on an LC.

Given:  1. HPLC instrument/ 2. C18 reverse phase column/ 3. 1mM solution of each of the following compounds in Methanol/ 4. Mobile phase is methanol-buffer mixture:

Compounds are 3 seperate groups:  
1. Carboxylic Acids:
    a. Benzoic acid  b.Phthalic acid  c. Toluic acid

2. Amino:
    a. aniline b.4-aminotoluene c. orhto-    
       pheylenediamine

3.Amino Carboylic Acids:
    a. 4-aminobenzoic acid  b. 2-aminobenzoic acid  
       c.4-amino-2-methylbenzoic acid

I need a suggestion for the buffer and mobile phase composition to get the best separation.