I take pics of my greeting cards in a nice photo booth, but they are always light on top and dark on the bottom. Using my cell phone camera and a light gray background.
Your question is a bit hard to answer, due to the lack of detail. What you see is most probably caused by your lighting setup.
From what I read I understand, that you use 1 softbox at the top and 2 at the sides. This will create even lighting horizontally, but cause a brightness gradation from top to bottom.
Without seeing the exact scene, you are lighting, it is hard to provide a solution, but here are some things to try out:
- Adding a front/bottom light
- Using a white piece of cardboard or similar as a reflector from the bottom.
- Using the "Inverse Square Law" by placing the lights farther away. This will decrease the loss of light in the distance between top to bottom, evening out the gradation. However, the overall loss of light might be too much on constant lights. Plus the shadows will become harder.