According to the scene light, improve the sensitivity and ISO, and control the shutter speed above 80.

When the mode is set to A, the shooting will look at the data of the film. Increasing the sensitivity and lowering the aperture will make the depth of field thicker. If you want to blur the background, the focal length must be lengthened, if the aperture is lower than F7. 5D3 is a full-frame machine, so the professional machine will definitely not have a built-in flash.