Why Doesn't Vegeta Go Ssj3

Why can’t Vegeta turn SSJ3?

First of all, it should be noted that Vegeta couldn’t go Super Saiyan 3 in Dragon Ball Z. He simply didn’t have the physical capabilities at the time, as Super Saiyan 2 was the extent of his power. However, Vegeta was powerful enough to use it through the entirety of Dragon Ball Super.

Why is Super Saiyan 3 not used?

ssj3 is too much of a stamina drain. goku explains that somewhere in the buu arc. That form is really meant to take care of buisness, he cant just play around in it like he does with the other ssj forms. Thats why he barely uses it.