iMac bad memory bank three beeps

The beeps indicate that something is wrong with the RAM module or the connection to the logic board. If your machine is still under warranty then you should take it to an Apple store or a certified Apple repair technician. The steps to check for yourself would be to remove one module and reboot. If that works fine then it is the other bank as it connects to the logic board or the other module itself. To be certain, repeat the procedure with the other module and remove the first. Do all the possible combinations of modules and banks to ascertain whether it is a RAM module or a logic board issue. This is a lot of shut down/ restart , removal and replacement but this is what the technician will do . The RAM bay on your iMac is located on the bottom under the display; you must remove the small cover to remove the RAM module then replace it before you try a restart. Without the cover the machine will not restart. If you are uncomfortable digging into the guts of your machine then you need to get it into the hands of an experienced and qualified technician. This is just the beginning of the investigation. It might not be the RAM modules at all but something concerning the main power board.