According to Wikipedia, buffalo milk has 8% milkfat by weight. Cow's milk is listed on that same table as 3.9%, so I'll use that figure for consistency, though the number does vary both from cow to cow and by breed.
So, you need to figure out the portions in which you'd mix together whole milk and heavy cream to reach the same fat content as your buffalo milk.
What you want is a formula in which the amount of fat from the whole milk plus the amount of fat from cream equals the amount of fat from buffalo milk. The amount of fat from the whole milk would be 0.039x, where x is the amount of whole milk, and the amount of fat from the cream would be 0.3y, where y is the amount of cream. And finally, the amount of fat that you would have from the same quantity of buffalo milk would be 0.08(x+y). Putting it together you have 0.039x + 0.3y = 0.08(x+y)
Solve for y to get y=0.18 x, or 18% as much cream as milk. If your milk truly is 3% fat, this would be closer to 22%. Lets meet in the middle at 20%, since it's going to vary anyway.
What this boils down to is that, to approximate the fat content of buffalo milk, you'd use about 200mL cream for 1L of whole milk.