I have (3) crashed hard drives from which I want to recover any files
I can. I now use a XP pro machine.
The drives:
A: 1991 230Mb Quantum ProDrive LPS scsi drive (used on Quadra 840);
B: 1995 700Mb Quantum ProDrive LT scsi drive (used on Power PC);
C: 2001 40Gb IBM Deskstar ATA/IDE drive.

I know I can buy a "Weibetech" dock for the IDE drive and attach it via USB2. 
Will it appear as an external drive? Is there a combination dock that
will also transfer from the 50-pin scsi drives? Can I place a SCSI-IDE
Bridge between the scsi drive and the IDE dock, and port that whole
thing to my PC with a usb cable?

And what drivers/software would I need to get my PC to recognize the drives?

In each case I believe there are bad blocks in the boot sector of the
drive and that some of my data is still recoverable. I can use a
fairly recent G3 Mac laptop to transfer files from the old Mac drives.
I also have an old syquest box with a 4-pin power supply and scsi
ribbon that I might use to fire up those old Mac drives.

Obviously I am eager to learn how to do this myself. I also want to
avoid paying a few hundred bucks per drive to some service. Solutions
that require the fewest new pieces of hardware, etc., will be most
appreciated.

Try a professional data recovery company.  I happen to know of one
that I prefer that will offer a discount for multiple drives:
www.rewave.com (ReWave Hard Drive Recovery).  They also offer a free
evaluation, free shipping, and no data - no fee, which may come in
handy with more than one drive.

Good luck to you.