Two older Macs (G3 / Intel) need their drives erased. No install discs
Do you have a current Mac with a Firewire port? You can attach the old Macs1 to the new one via Firewire, and boot them using Target Disk Mode, and use Disk Utility on the new Mac to wipe the drives (they'll appear as external drives).

1 Note that the earliest G3 iMacs didn't have Firewire, and thus can't use this feature. The G3 with slot loading drive (released in Octoboer 1999) and later will work.