>>531227572
You have a 1/5 chance for any individual stat to be the spark. At 600+ in a stat you have a 5% chance to get a 3* spark for that stat, if it rolls.
So basically just do career runs, make sure to hit 600/600/600/x/x, and pray. Horses that do medium and long racers are best suited for this stat line. Gold ship is an easy one to run.
There's also a breakpoint at 1100+ on a stat for a 10% chance but that's virtually impossible for any stat except speed so we ignore it.