I had a similar occurrence, I did a few High Heat races before I got to 50, and didn't get any Ultra/Ultra+ parts for finishing in top 3. I think you must be level 50 in order to be able to receive them.
However, once you are Level 50, it is essentially a lottery. You will unlock the components at random, regardless of which ones you already have and don't (you will receive duplicates).
Keep in mind, you don't have to finish in the top 3 to get the part. I have had a race where I finished 4th/5th and still got one. In addition to this, the only difference between the Heat 3 and Heat 5 race seems to be the drop chances for Ultra+, although it doesn't look like a big difference. I've gotten quite a few Ultra+ from Heat 3s.