What you could do it get the 4MB RAM and ROM, install it and use it, then when you get the other 12MB install that. That would be fine. You will see what I mean when you do the initial upgrade.
To line up the screws is wasy. Place the A3000 face down on a padded surface like a clean carpet, then press down hard while inserting the screws and screw them in. It may be a little tough but it can be done. (I would not do that until you get the final stuff in though)
Good luck with your Amiga 3000. It is a great machine. Just make sure you remove the battery and clean up the board if it even hinted of a leak. The Amber is in that area and will cause video problems or worse.
