Best way to photograph A3 paper paintings table top?

Should be okay. Depends what the image is like. I always use a 60mm for flat lay reproduction files.

Clarification: A 60 mm lens for art or page / document reproduction. The lighting can be highly variable based on what your end effort is. Photigy contains hours and hours of tutorials here that I've gleaned tips over the years for these types of shoots. But for the lights - I'll usually use modifiers with grids and flag as required.

 

If you want the cleanest photos, mimic the setup of a copy stand. Two softboxes either side angled at 45 degrees or shallower and you won't get glare coming off the paintings. if they are on paper, they might have curled slightly, so in that case, get a really clean sheet of glass and put it on top of the paintings (unless they are textured). Same light setup won't result in reflections and the paper will stay flat. as for focal length, stay between 50-100 to minimise distortion.

oh: and photograph a colorchecker at the beginning of the session to maintain colour accuracy.

hope that helps.