A free, basic prefab paint for Unity3D.
Import the project into Unity and open the prefab brush by clicking Tools>Prefab Brush.
ctrl + left mouse button to paint
ctrl + scroll to change the brush size
alt + scroll to change the brush density
FreePrefabPainter has been tested with Unity 2017.3.0b10.
https://forum.unity.com/threads/released-free-prefab-painter-github.506118/