My friend’s horse is injured in the race.

The given statement describes a horse that my friend owns.

Hence, the word friend’s is a singular possessive noun with an apostrophe before s.

The doctors' schedules are canceled for today.

The given statement describes the schedules that belong to the doctors.

Hence, the word doctors' is a plural possessive noun with an apostrophe after s.

Sam’s whistle was lost while exiting the game.

The given statement describes a whistle that belongs to Sam.

Hence, the word Sam’s is a singular possessive noun with an apostrophe before s.