The 3 different turbos are NOT cumulative like the engine tuning or weight reduction are.
You do NOT get more total HP by buying all 3 turbos.
Each one gives you additional boost in a specific part of the rev range and you must choose which one will be most effective for you. You can, of course, buy them all and then choose which one to run dependent upon the needs of the track you're on. But you only get to run one at a time.
Buying all three and then always running the High RPM one is just wasting 14,500 cr.
Not sure if your post meant that you bought both of them to be on the safe side so that you'd have the option depending on the course, but I don't want anyone else thinking that you need to buy them all to get the most HP boost.