Hello,
it seems like some/all objects in my scene are using 3 draw calls with the diffuse shader.
This is unexpected because I only have one camera and one directional light. So I’m expecting at most 2 draw calls per mesh.
Is there anything that could be causing these extra drawcalls?
See if the RenderMode on your lights is at Auto (which uses the QualitySettings) and try setting it to Force Vertex. I believe that the if you have the QualitySettings set at default it will use the Pixel lighting which could be the extra draw calls you see.
Pixel lighting with 1 light should be 2 drawcalls per mesh, correct?
Still getting 3, no idea why. 
Do you have shadows enabled on the light? Those will add at least one draw call per object per light.
Does “Fog” cause extra drawcalls?
Oooooh wait… maybe it’s a blob shadow projecting itself on the mesh… it cause an extra draw call per mesh it hits, right?
That’s right. Each mesh in the projector’s frustum is re-drawn with UV coordinates based on the projector’s location with respect to the mesh.