If this hypothetical fusion ability worked as if the Pokémon had all three abilities at the same time, yes, that sounds like the correct result.
However, as you may gather from the fact it's so hideously broken as to give it a 156-power Quick Attack, if there were ever a real Mega Pinsir, Mega Beedrill and Mega Scizor fusion, it would not be given an ability that works like all three of its source Pokémon's abilities at once. There is a reason Pokémon only have one ability at a time: stacking two or three conditional damage-boosting abilities together just gives absurd results. If you're trying to design a remotely believable fake Pokémon, this is not the way to go.
