DIAGNOSIS That three-beep sequence is the mower's safety system telling you something isn't right for startup. The most common culprit is the seat sensor not registering your weight.
FIX First, make sure you're fully seated and not leaning to one side. If it still beeps, here's how to check the sensor connection. 1) Park safely and turn the key off. 2) Lift the seat. You'll see the wiring harness plugged into the sensor pad underneath. 3) Unplug it, check for any debris or moisture in the connector, and plug it back in firmly. 4) Sit down fully and try again. Often, just reseating the connection fixes it.
IF THAT DIDN'T WORK Check that both drive control levers are pushed fully out into the neutral position. If they're even slightly inward, the mower thinks you're trying to start it in gear, which triggers the safety beeps. Push them outward until you feel them click into the neutral detent.
HEADS UP Don't bypass the seat sensor by jamming something into the connector. The mower will drive with the blades engaged if you get off, which is a serious safety hazard. Fix the sensor the right way.