matching contrast on stitched images

[betwixt1]

Can anybody suggest a quick way of fixing this problem in PI12 to save me using trial and error:

A local artist asked me to photograph a painting he made before delivering it to his customer. Time was very short and all there was to hand was a digitial camera - no tripod or specialised lighting. The canvas was 5' tall and 7' wide (about 1.75m x 2.1m) and the room was too small to get far enough away to get the whole image in one shot. By maneouvering the camera and canvas, I managed to get it in three overlapping shots. There is about 40% overlap of the left and right images over the middle one. Unfortunately, in moving things around there was some degree of geometry shift and lighting shift between the shots. I fixed the geometry by stretching and tilting the images so the frame edges are squared up but lighting shift is a more elusive problem. Probably due to the canvas being tilted forward or backwards slightly, the illumination on the shots is slightly different. When stitched there is a noticable border where the edges meet. It is progressively worse as the stitch goes up from the bottom. Is there an easy way of adjusting the brightness/contrast/saturation so it tracks the neigboring image? I'm guessing some sort of gradient mask needs to be made and applied to two of the shots to make them match the third but I can't see a way of creating it without resorting to progressive guesses. Thankfully, the painting is of a mountain scene with sky at the top and fields at the bottom so there are easy areas to match to.