In the juggler's lingo the word flash can mean two things. The term is used in numbers juggling to describe throwing & catching everything once. So a seven ball flash would be: throw, throw, throw, throw, throw, throw, throw, catch, catch, catch, catch, catch, catch, catch. The second meaning is to juggle, throw everything up high so that both hands are momentarily empty, then catch & resume juggling. This trick is excellent practice for the five ball cascade.
To throw a three ball flash you start off with a High Throw, but don't wait for it to peak before you throw the next ball, if you do you'll go into a high cascade. Instead throw it immediately afterwards & the same with the third ball. just count out '1,2,3, gap, gap, catch, catch, catch'. Simple. If you want to be very precise, your second throw should be made at the point where the first ball would've peaked if you had thrown it normally.
Practise the flash leading from both hands, a right hand lead flash is 'right, left, right' & a left hand is 'left, right, left'. Also practise this trick throwing to varying heights. When your hands are empty clap them together. Try clapping once behind & once in front of you. Then try clapping once behind, once in front & once behind again.
A common & flashy move to try under a flash is a pirouette. Do three high throws, spin round & then resume juggling. This can be done indoors under a ceiling (I'm 5'11" & my ceiling is 7'6"). Start off juggling as low as possible then throw all three as accurately as possible. Wait until all three balls are out of your hands before you start to turn. Kick with one foot, spin on your heel & stamp down to stop, don't just shuffle round. I also judge where to catch the first ball before I spin because there is not enough time to spot it between when I complete the turn & the time I have to make the catch.
Not enough for you? Try a double pirouette, or three or more.