I think 6MP is more than enough for A3. I know because I have printed a number of 16 inch by 20 inch prints using the humble D70 and the results are tack sharp, even at close range. And those were shot with the kit lens. I feel the result could have been even better if say a sharper lens like the 50mm 1.4 was used instead.
6 megapix is definitely enough. I've done a job that printed the output from a D70 to a wall picture about 6m on it's longest edge. viewing distance being about 1m away, no pixelation of output could be seen. Someone even commented if it was painted.
