RE: Inspiron 14z 5423 3 Beep Code
Unplug the system, disconnect the battery (requires removing the base cover), and hold the power button for 30 sec. Remove and reinstall both memory modules.
Try powering up again - if you can get past the three-beep halt, proceed with the diagnostics as above. If you cannot, as noted your mainboard has failed and will need to be replaced. No further diagnosis is possible at that point - and since just about everything inside the system is soldered to the mainboard except the memory and drive, there's little you can do other than replace it.