I think Goku would maybe survive it considering he already survived hits from beings many times stronger than him (e.g final form Freeza), but he surely would be rather injured, even more so by the FF on top of SSJG3. Vegeta is over half Goku's battle power, so even though Goku is way stronger than Vegeta the gap still has a cap on it. We don't have a precise accuracy on the SSJG2's amp despite the fact that Piccolo stated "his chi is quickly increasing" and then "it's still increasing more and more", denoting a pretty big power up, I'd say. It's true that even a 1.3x power up can be seen as a pretty big increase considering Vegeta vs Cui, but I tend to see it as a 1.4~1.5x increase considering that not even Cell's power up, said by Vegeta to be "quite a power up" was enough to turn the battle into a competitive one, and Vegeta still tanked Cell in a similar way he did #19. A 1.4x increase also fits my theory that Freeza powering up from 70% to 100%, Cell powering up to FP and Vegeta SSJG2 are all similar increases. So, to illustrate:
Vegeta [CG] : 66
Vegeta grade 2 : ~ 92
Goku [CG] : 100
As for the Final Flash, Cell was completely stomping Vegeta, to the point him tanking Super Vegeta's kick was one of the most impressive tanks next to Freeza tanking Nail and Cell tanking #16. Yet, the Final Flash was in all likelihood considerably above that Cell, who'd be screwed if not for his regeneration. Considering that Cell was way above Vegeta and that the Final Flash was considerably above Cell, I think we can safely say that the FF amplification was at least 2x.
Vegeta SSJG2 + FF : ~180
Goku [CG] : 100
So Goku would be already in a rather big disadvantage. As for the Grade 3 amplification, we know Trunks with such power was labeled as "quite above" or "easily surpassing" Cell, to the point Trunks seemed sure about his victory when not knowing about the speed weakness. So, considering that power-wise Trunks is considerably above someone that could easily tank his 'lesser' form, I could easily see that as a ~2x power up on top of the SSJG2 as well, making Goku multifolds weaker than a hypothetical SSJG3 + FF Vegeta:
Vegeta SSJG3 + FF : ~360
Goku [CG] : 100
That would be it as far as my estimations go, in which Goku would be already rather outclassed by a SSJG2 + FF, nevermind a grade 3 + FF. But yeah, considering base Goku was already enough to take some hits from Freeza and that Goku is known for his brutal durability, I have my doubts if he'd be killed by such an attack. Injured of course, but it's possible he is still breathing after that, of course. May be also worth noting that Goku's durability might be seen sometimes as plot based and when the plot calls for it he can survive attacks from someone he normally wouldn't.
vs Fat Boo (1st power up)
