Diagnosis
The triple beep with flashing lights is this model's standard error indication for a water supply issue. The machine isn't getting enough water, or it's not detecting that water is flowing in.
How to Fix It
First, check the obvious: make sure the water tap under your sink that feeds the dishwasher is fully turned on. Then, check for kinks in the inlet hose behind the machine. If those are fine, the inlet valve filter might be clogged. You'll need to pull the machine out (turn off power and water first), disconnect the inlet hose, and check the small mesh screen where the hose connects to the machine. Clean any debris off it.
If That Didn't Work
The water pressure in your home might be too low for the dishwasher's sensor. Try running a tap on hot for a minute to clear the line, then try the dishwasher again. If it's a recurring issue, the water inlet valve itself could be faulty.
Heads Up
Don't ignore this error and keep trying to start it. The machine will time out and stop trying to fill, which prevents flooding, but you need to solve the water supply problem first.