My iMacG3 starts up then after 2 or 3 minutes it hangs.

(shift) boot to determine if it's an extension you recently added. Once "in" launch Applications->Utilities->Disk Utilities (DU) and run Disk First Aid (DFA) Repair Permissions, and then run DU:DFA: Verify Disk. I (corrupt permissions can cause strange application behavior) if disk verify tosses out an error you'll have to boot from an install disk and run Tools->DU:DFA:Repair Disk if that fails your HD is likely going.

You don't mention so we don't know if you recently installed RAM, an update or application (which failed) to kick off this behavior, had any power surges/failures or have any free space on the HD (at least 20% is required to be free to run temp and VM files).

The more we know the more we can offer in the way of help.

Remember to return and mark as accepted the answer that best solves your problem.

Good luck,

N