TQQQ aims to return three times the Nasdaq-100's move each day, not each year. To do that it rebalances every evening. After a rise it borrows more; after a fall it sells. That keeps the daily multiple exact, and it costs money whenever the market goes back and forth.
Take a day when the index rises 10% and a day when it falls 9.09%. The index ends where it started. A 3x fund rises 30% and then falls 27.27%, and ends down 5.5%. Nothing went wrong; that's the arithmetic of resetting daily.
Over a year the drag is roughly three times the index's variance. At 20% volatility that's about 12% a year. At 40%, about 48%. The drag doesn't care about direction, so a volatile sideways market can halve the fund while the index ends flat.
Over the full study, holding a TQQQ-like fund compounded at 14.7% a year against 14.9% for plain QQQ. That's three times the risk for the same return, with a worst drawdown of −99.98%. The leverage only pays when volatility is low, which is the reason the signal exists. Try the calculator