A B737 AT FL410 ENCOUNTERED TURBULENCE ABOVE A RAPIDLY DEVELOPING TRW RESULTING IN THREE FLT ATTENDANT INJURIES AND A MACH AIRSPEED EXCEEDANCE.

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FLYING AT ALT WE WERE DEVIATING AROUND AREAS OF ISOLATED TSTMS. A PERIOD OCCURRED THAT NOTHING ON THE RADAR WAS APPEARING EXCEPT FOR AN AREA BEYOND 80 MI. FLYING ALONG; THE CAPT NOTICED AN AREA AS HE TILTED THE RADAR DOWN THAT NEEDED A DEV IMMEDIATELY. AS WE WERE ASKING ATC; WE ENDED UP IN IT ALMOST WITHIN JUST A FEW SECONDS. THE BEST COURSE OF ACTION WAS TO FLY STRAIGHT AHEAD AND RIDE THE FLT THROUGH THE TSTM TOP. DURING THE RIDE WE EXPERIENCED MODERATE TURB AND A MACH EXCEEDANCE OF .86 FOR LESS THAN 7 SECONDS. SEAT BELT SIGN WAS ON AND THE 3 FLT ATTENDANTS WERE IN THE AFT GALLEY WHEN EVENT OCCURRED. ALL 3 FLT ATTENDANTS WERE INJURED. NO PAX INJURIES WERE RPTED THAT I AM AWARE OF. FLT CONTINUED TO DEST AND MEDICAL TREATMENT TO THE 3 FLT ATTENDANTS WAS ADMINISTERED. OUR BELIEF IS THAT THIS TSTM WAS WELL BELOW THE SCAN OF THE RADAR AND WAS BUILDING AT A VERY GOOD RATE OF 5000-6000 FT PER SECOND. BY THE TIME IT WAS NOTICED; WE JUST HAPPENED TO BE ON TOP OF IT. A POTENTIALLY BETTER SCAN OF THE RADAR WHEN AREAS ARE DEVELOPING TSTMS AND TAKING THE BEST COURSE OF DEVS NECESSARY. IN MY 23 YRS OF FLYING; I HAVE NEVER HAD AN EVENT LIKE THIS OCCUR. I HOPE IT NEVER HAPPENS AGAIN.