I'm assuming that the numbers you gave (90, 87, 96) are based on a system of 100 possible points (?) If so, then you you merely add each of the numbers together, in this case 90+87+96 = 273, and divide by the number of grades, in this case 3. Dividing we get 273/3 = 91. This would be your average score based on a system of 100 possible points.
If the grade you are trying to calculate is based on another system, for instance if the class had a total of (arbitrarily) 290 points, then we would calculate as follows.
Add the points together, 90+87+96 = 273, now divide by the total points, 290. We come up with 273/290 = 94.138%
We can then multiply this result times the value for an "A", in many systems this is 4.0:
0.94138 * 4 = a GPA of 3.765