Student Grade Calculator

You are tasked with completing a C# method CalculateStudentGrade that determines a student's final grade based on three scores.

The method should:

1. Calculate the average of the three provided double scores: score1, score2, and score3.
2. Based on the calculated average, assign a letter grade according to the following rules:
   • If the average is 90 or above, the grade is "A".
   • If the average is between 80 (inclusive) and 90 (exclusive), the grade is "B".
   • If the average is between 70 (inclusive) and 80 (exclusive), the grade is "C".
   • If the average is between 60 (inclusive) and 70 (exclusive), the grade is "D".
   • Otherwise (below 60), the grade is "F".

Complete the missing logic and correct any potential errors in the provided CalculateStudentGrade method.