iMac 3 on beeps, black screen

When trying to turn it back on 3 beeps sounded and did not turn on more.

After a few days, I turned it on normally, it was OK. On another day, the problem came back: while I was using it stopped working and did not turned on, just sounds of 3 on 3 beeps.

The screen does not turn on, goes black and no other sound is emitted beyond the beeps.

I've done testing of the RAMs, but nor replacing them it works.

Has anyone had this problem and managed to solve?

Apple Lab

ADC Empire Plaza, 2nd Floor 183, Satmosjid Road,12/A Dhanmondi

Dhaka 1205