If you want equal parts fert and bone meal, 1.5 cups of fert and 1.5 cups of bone meal. 1.5 + 1.5 = 3.0

Dan

I will take a guess that you may have blood meal, not bone meal ?
If it is 12% N that sounds like blood meal.

If I am not mistaken, if you mix equal parts of your 3-4-4 and the 12% N fertilizer you would get 15-4-4 ( assuming it is 12-0-0 )

What are you trying to achieve? You have 3-4-4 and you have what appears to be 12-0-0 (you called it bone meal, but to be 12% nitrogen it is probably blood meal).

You said you want "a total of 3 cups (equal parts)". By "equal parts" do you mean equal amounts of N, P, and K?

If so, you can mix 1 part of 12-0-0 with 11 parts of 3-4-4 to achieve 4-4-4. To get 3 cups worth, mix 1/4 cup of 12-0-0 and 2 3/4 cups of 3-4-4.

I agree with all of the above. You cannot have bone meal that is 12% N. So what do you really have.

Second, your question, your goal isn't at all clear so what are you trying to accomplish and why?

Dave