DIAGNOSIS The triple-beep is the mower's safety system yelling at you. It's almost always because it doesn't see the battery properly seated or it thinks the battery is too low to safely start.
FIX First, pull the battery all the way off the mower. Look at the battery contacts on both the mower and the battery for any grass clippings or dirt. Give 'em a wipe. Then, really slam that battery back in until you hear a solid, definitive CLUNK. Don't be gentle—it needs to fully click into the latch. Now try the start handle again.
IF THAT DIDN'T WORK The battery itself might be the problem. Try a different 40V Ryobi battery if you have one. If it works, your original battery is likely discharged below the mower's safety threshold or has a bad cell. You'll need to charge it on the regular charger; if it won't take a charge, it's probably toast.
HEADS UP This mower is picky about battery voltage. If you try to start it with the battery showing just one bar, it'll often triple-beep and refuse. Always start with at least two bars for a reliable crank.