In this case, Metformin is given for Polycystic Ovary Syndrome (PCOS) unlikely for diabetes. PCOS usually happens in women who are overweight. Perhaps you might want to advise your wife to lose some weight before trying to conceive again.

You did not mention about Haemoglobin (Hb) level during the three miscarriages. Low Hb level will cause less red blood cells in body to carry oxygenated blood to the embryo and consequently lead to miscarriage. As mention in previous post, just add iron supplement (such as IBERET) can help to increase Hb level in the body. However, remember to avoid taking calcium and iron supplement at the same time because it can cause poor absorption of iron.

Just my humble opinion.