Who is Vegeta to Goku?
They aren’t brothers. Vegeta was the prince of the Saiyan planet before it was destroyed by Frieza. Goku is the son of a Saiyan warrior Bardock who died along with their planet. Goku had an elder biological brother, Raditz.
Why did Vegeta turn Super Saiyan 3?
The reason Vegeta could’nt go SSJ3 is because Goku is a Honour Gene. Honour Gene’s can go SSJ3. However, Vegeta wasn’t a Honour, instead he is a Pride Gene. Pride Gene’s can only go SSJ, ASSJ, USSJ, SSJ2, and SSJ4.