Late to this - off the top of my head, there used to be an 'easy' way to do this prior to install - post-install, it meant changing the preferences/options or a .*.ini type file to add the paths of the folders containing the data from each CD.
IIRC, make a new folder called BWP, inside that folder create 3 new sub-folders, CD1, CD2, CD3. Copy the contents of each CD to the relevant sub-folder, run the exe/installer from inside CD1 and when/if the install routine prompts for the location of CD2, point it to the CD2 sub-folder, etc.
For your already installed solution, simply create and populate the 3 sub-folders in the existing BWP folder and modify the Options/Preferences (or was it the ini file?) and you should be good to go.
When I find my BWP CD's or old drive, I'll check.
For anyone wishing to try this with recent Windows versions (I never did install it on my G3 'book so I've no idea of how it installs on OSX), you're likely to run into a autorun/install problem with the BWP installer (incompatible 16bit Macromedia Projector part of the installer, iirc), it can be bypassed in W7 but I don't recall the exact steps I used.
3rd party virtual drives, as suggested by Phil, are a decent workaround but may lead to 'random' bluescreen crashes further down the line.
