Diagnosis
The three-beep error on many Frigidaire models is a water-related fault. It usually means the dishwasher isn't filling with water, or it's taking too long to fill. The most common reason is a clogged or kinked water inlet valve screen.
How to Fix It
1) Turn off the water supply valve under the sink (usually a braided hose going to the dishwasher). 2) Disconnect the water supply line from the inlet valve on the bottom left of the dishwasher (have a towel ready). 3) Use needle-nose pliers to carefully pull out the small cylindrical screen from the valve inlet. It will likely have sediment buildup. Clean it under running water. 4) Reinsert the screen, reconnect the hose, turn the water back on, and check for leaks before running a cycle.
If That Didn't Work
The water inlet valve itself might have failed electrically and isn't opening. You can check for 120V at the valve terminals during a fill command, but that requires a multimeter and some know-how.
Heads Up
Always turn off the water AND the electrical power before disconnecting any hoses. That inlet valve has live wires connected to it even if the unit is off at the breaker, unless you unplug it.