The size at 3 years of age can depend entirely on the particular variety. In this case, since it's blooming, all would appear to be well. 2.5 feet at 3 years would give a 10 year height of 7.5 feet which would be pretty normal, depending on the size when first planted.

Actually many of the early-blooming lepidotes don't get very tall. It might max out at 4 to 5'. Rhodies are usually a couple years old when you buy them if you bought it in bloom.