Are you referring to doing fasted cardio? If so, doing that while trying to gain muscle mass is about the worst idea I've ever heard.
Just eat a moderate caloric excess if you want to mitigate fat gains while trying to pack on muscle mass. Try a 10% increase in calories over maintenance as a starting point.
Also, whoever told you that you burn 3x more fat when doing fasted cardio is by my best guesses, misguided. They probably saw a poorly conducted study that suggested something along those lines and ran with it. Furthermore, total calorie balance is still most important. That type of thing may be of possible use when trying to remove fat once you are already relatively lean. When trying to add muscle mass it is just completely and utterly silly.