When they used it, Goku charges the Kamehameha, Future Trunks charges the Galick Gun, and Vegeta charges the Final Flash. Once fired, the three energy waves combine into one, inflicting a great amount of damage.
As Infinite Zamasu was spreading throughout Earth's atmosphere, Goku, Vegeta, and Trunks decided to use this attack against Infinite Zamasu when they had no other option. Since Goku and Vegeta couldn't transform into Super Saiyan Blue, due to their fusion's usage of the Final Kamehameha, Future Trunks goes Super Saiyan and used this on Infinite Zamasu. This was to no effect, due to Zamasu's immortality.