I have the exact same scanner up here at work and I agree it is a total pile of crap. We needed one to scan in some tech documents for some older equipment and that was pretty much the only one that we could find and honestly it would have been better to just redraw and retype out all the documents and not deal with that thing. We ended up taking the bulb out of it and using it as a desk light for one of the work benches. Best use of any Mustek product. Because everybody buys them and then realizes they are crap but you are already stuck with it.
If you're only scanning A3 stuff, just use an A4 scanner and Photoshop or gimp. Make sure there's enough overlap, use an alpha mask, and viola. Takes a while to mere them at huge DPI but it's possible.