Does Vegeta knew that Trunks has Grade3?

[Let's Go Fearless!]

Is it possible that he knew Trunks could transform into G3 mode but he doesn't care because its useless anyway?

 

[Jerk Store]

Who knows. What we do know is that Vegeta said Trunks was almost as strong as him, so if grade 2 Vegeta is stronger than grade 2 Trunks, then roid rage Vegeta > roid rage Trunks as well. Trunks realized the forms flaw and understood why his father didn't use it. So Vegeta probably reached the form and opted not to use it and knew Trunks would be an idiot nublet if he used it.

 

[Let's Go Fearless!]

Or probably he didn't expect Trunks to used that given its speed loss.

 

[Tapion]

I don't think so.

 

[sei'taer]

He probably didn't expect Trunks to be that stupid.

 

[SSJ2]

Doubtful. In fairness Gr 3 isn't really even a separate form to Grade 2, it is just going beyond Grade 2. So as sei'taer said, he probably didn't think Trunks would be such a dumbass and continue to increase his power.