A Basic Code Wars Challenge

Hi everyone this is one of those times, where the solution is staring me right in the face but I can't seem to find it! So please be patient with me: The instruction is the following:

Complete the function so that it finds the mean of the three scores passed to it and returns the letter value associated with that grade.

Numerical Score Letter Grade

90 <= score <= 100  'A'
80 <= score < 90    'B'
70 <= score < 80    'C'
60 <= score < 70    'D'
0 <= score < 60 'F'

Tested values are all between 0 and 100. Theres is no need to check for negative values or values greater than 100.

Here is my solution:

function getGrade (s1, s2, s3) {
  var score = (s1 + s2 + s3) / 3;
  if (90 <= score && score >= 100) {
      return 'A';
  } else if (80 <= score && score > 90) {
    return 'B';
  } else if (70 <= score && score > 80) {
    return 'C';
  } else if (60 <= score && score > 70) {
    return 'D';
  } else if (0 <= score && score > 60) {
    return 'F';
  }
}

getGrade(5,40,93);
getGrade(30,85,96);
getGrade(92,70,40);

Can't for the life of me figure out what I am doing wrong.