The cloverleaf is a really important and historic symbol for Alfa Romeo. In the 1920s, one of Alfa's racing drivers was Ugo Sivocci. He was a very good driver, but somehow he couldn't win any races. He was very unlucky, and considered the "eternal second". For the 1923 Targa Florio, in Sicily, he had a cloverleaf, inscribed in a white square, painted on his Alfa Romeo "RL Targa Florio":
At the end of the race, he was second, while Antonio Ascari, on another Alfa Romeo RL Targa Florio, was first. But, at about 200 yards from the finish line, Ascari's car failed. The mechanics ran to fix the fault, then hopped on Ascari's car, and crossed the finish line with him, still in first position, as he was so far ahead. But Ascari was disqualified, since he couldn't race with his mechanics onboard. So he had to return to the point where his car broke, and drive to the finish line again. But by then, Ugo Sivocci managed to cross the finish line, winning his first race. The cloverleaf worked!
The same year, a few months later, Ugo Sivocci was testing the new Alfa Romeo P1 in Monza. That car didn't have cloverleaves, as the team personnel didn't have the time to paint them on. He crashed and died. From that day, all racing Alfa Romeos had the cloverleaf as a tribute to Ugo Sivocci, now inscribed in a triangle rather than in a square to signify the loss of one of the four drivers of the Alfa Romeo team.
BTW, the P1 he was driving when he died had 17 as its racing number.