There is a multitude of variants how to prepare the target buffer especially taking into account that it is unnecessary to use all 3 unknown compounds. It is evident that A and B are the NaH2PO4 and Na2HPO4 respectively (exact molar masses and pKa of HPO4- anion). So the easiest way of solving the task is to refer to the standard protocols to prepare the phosphate buffer solution with pH7.6
For example Cold Spring Harbor Protocols give the following recipe:
15.5ml 1M solution of A + 84.5ml 1M solution of B results in 100 ml of 1M buffer pH7.6
To obtain the 0.1M solution 10ml of 1M buffer should be diluted with water and the volume adjusted to 100ml. It is important to note that dilution slightly interferes the pH, so it is necessary to adjust the pH by adding a few drops of sodium hydroxide or phosphoric acid solutions available in the laboratory, and to control the pH by using the pH-meter.