Marcos Daniel, you can try overclocking the RP3 with just a heatsink. Monitor the temperature of the CPU so you can see how much heat the RP3 is generating due to the overclock and if the CPU temperature gets close to 82 'C (ie. 78'C, 79'C, 80'C, etc) then you will need a fan for the RP3 CPU. Overclock it following the instructions in the tutorial, open Pianoteq 6 with my settings on the tutorial (48KHz internal sampling rate, etc.), play something, and then type the following command in the terminal to check the CPU temperature:
/opt/vc/vcgencmd measure_temp
This will give you a good idea of what the temperature will be while you're using Pianoteq 6 and if you need a fan or just a heatsink. Also, try to fine tune the Pianoteq 6 setting to what your RP3 can do. Yours could be better than mine and achieve better results.