how to reinstall macos by flash drive on imac g3

You are going to need the original recovery discs (optical discs) that came with that Mac – or a retail copy of some version of Mac OS X, on optical disc, that is compatible with it.

PowerPC-based Macs predate Recovery Mode and Internet Recovery.

Apple doesn't offer downloadable full installers for any version of Mac OS X that will run on a PowerPC-based Mac. You might find, e.g., downloads to update a copy of Leopard to the latest release of Leopard, but this would require you to have some release of Leopard installed first. It wouldn't be a substitute for recovering using an optical disc – or a bootable external disk made before the internal Mac OS X installation became unusable.

a G3 iMac is way too old to support internet recovery...

The OP is going to have to dig up the original disks that came with it to reinstall OS X.

I concur with KiltedTim...

Insert the original OS Disc or a compatible OS X Upgrade Disc into the CD slot.

1. Start up from macOS Recovery. 
2. While in Recovery, use Disk Utility to repair your startup disk. Then proceed based on the results:

• • If Disk Utility finds no errors or repairs all errors, restart your Mac by choosing Apple menu  > Restart. If the question mark returns after restarting, start up from Recovery again, then reinstall macOS from Recovery.
  • If Disk Utility finds errors that it can't repair, use Disk Utility to erase (format) the disk. Then reinstall macOS from Recovery.
  • If your startup disk doesn't appear in Disk Utility, or Disk Utility reports that the erase process failed, your Mac or startup disk might need service. Shut down your Mac and unplug all nonessential devices, then try once more. If your startup disk is an external storage device, check its cable, connections, and power as well.

If the question mark appears only briefly before your Mac starts up normally, make sure that your startup disk is selected in Startup Disk settings. If that doesn't help, reset NVRAM and check the setting again.