Yep. 1st number is the tread width. In other words, how wide the tread is.
2nd number is an aspect ratio. For example...
If your tire size is 200/50/16 (not a real size I think, but easier to do math on), then the tread width is 200 millimeters. The side wall can be figured by thinking of the middle number as a percentage, and multiplying it by the first number. 50% of 200 millimeters, means that your side wall is 100 millimeters tall, or wide.
And yeah, the 3rd number is your rim size.