>>720007074
Each pull has a 3% chance of giving you a 3*. The chance of getting the rate-up horse (Seiun Sky in this case) is 0.75%, and the remaining 2.25% is split evenly between every other 3* in the game
If I'm remembering my probability equations right, that would math out up to roughly a 26.26% chance of getting any 3* on any given 10-pull and about a 7.25% chance of getting Sei specifically