The three beeps indicates a memory issue. Many times the memory slot nearest the bottom case fails due to bad solder joints, but the other memory slot can sometimes fail although it is much more rare. You can try reseating the memory since the latches holding the memory in place sometimes will pop loose. If this doesn't help, then try using just a single stick of memory. Try that single stick in both slots. Then try the other memory stick by itself in both slots. See if you can determine which memory stick or slot is bad. It is rare for both memory modules to fail.
Make sure to disconnect both the power adapter and battery whenever inserting or removing parts like memory.
You may want to try disconnecting the battery and powering the laptop on with just the charger. I have seen the sleep LED blink with some types of battery failures. I don't recall if the system beeps when this happens. Have you been able to charge the battery (check the battery indicator lights on the side of the laptop to see if more than one LED lights up).
FYI, never let the battery remain completely discharged for any length of time as it can cause permanent damage to the battery. A completely discharged battery can also prevent a Mac from powering on.
