Burn a .dmg file on a pc, have it bootable on my g3

Why bother if you have the Tiger DVD? If your G3 has a DVD drive you can boot it from the Tiger DVD.

If your PC can make track copies and if the .dmg files you made are "pure" copies, then the answer would be Yes. However, if you used PC software to make the .dmg files then there is a potential problem that the images are themselves not Mac compatible. Since I know nothing about how the images were created or with what software, I can't give you a definitive answer. Of course you can simply try burning the image to a DVD and then trying to boot the G3 with it.

A "track" copier basically duplicates everything on the disc verbatim regardless of format or content.

Instead of what you did try inserting the disc in the optical drive, restart the computer and after the chime press and hold down the "C" key until the computer starts up from the CD (gray screen with dark gray Apple logo.) Release the key when the spinning gear appears.

If this doesn't work, but you can start up from the original CD, then you know that your PC is not making a "pure" clone of the CD.

I don't use Windows so I have no idea what would be the best software to use for the task. There is software that enables Windows to read and write Mac compatible disks - MacDrive from MediaFour.

They should probably be .cdr (CD/DVD master) instead of .dmg.

From Disk Utility help:

"If you want to use the disk image with a third-party application, choose CD/DVD master. The disk image contains a copy of all sectors of the disk, whether they're used or not, and copies them to other CDs or DVDs bit for bit."

I guess that means they are not going to work as copies of the installer CDs.

Do you have the facility to make the copies on a Mac? That would be your best bet. You can make copies on a Mac as follows:

Duplicate a CD or DVD

1. Insert the DVD/CD;
2. Open Disk Utility, and select the DVD/CD from the left side list (select the DVD/CD icon on top);
3. from the DU File menu select New | Disk Image from Disk 1;
4. Choose to format the disk image as DVD/CD Master, name the disk image and click Save;
5. When the .cdr file is finished select it with mouse and press COMMAND-I to open the Get Info and check the box to lock the file;
6. Choose the .cdr file from the left side list, click Burn, and insert a new, blank DVD or CD.