Ok, he didn't react to Gotenks. But part of your point was "they were all dumbfounded by 50% Goku so >> grade 3 Trunks", right? These reactions don't show much when the people who witnessed Goku SSJ3 were also dumbfounded by base Gotenks, who should be <<< fat Boo (Piccolo already knew it from the beginning yet was :shock.
He should have, but he dismissed it. Trunks has thought about Grade 3 as being his true power for a long time. It's not like he automatically forgets what that level is like after just recently being told it was useless.
The scene is much more dramatic if Trunks is surpassed in his highest form. And Buff Cell, who is at least Trunks equal, if not stronger.
He dismissed it because he knows it's useless. He would surpass Cell's power with grade 3 and yet was completely surprised at Cell's showcase of power. Why? Because he knows grade 3 is an useless power and he won't be taking it into consideration. Trunks was already schooled twice times (both by Vegeta and by Cell) that that type of transformation isn't worthy doing, so I don't think he has to be considering it in his reactions, just like Vegeta wasn't. That's not Trunks' true power anymore, nobody considers this form his true power:
Chapter: 409 (DBZ 215), P3.5
Context: after Gohan says he can beat Cell
Cell: “Well then, I’ll show you…! My fearsome true power…!”
Look at Trunks in the Cell Games as another example. He should be able to perform the grade 3 transformation and he was ahead 50% Goku. I'm sure you think the grade 3 transformation is a bigger boost than 2x, which would put him way ahead 100% Goku. Yet how Trunks reacted to 100% Goku? He was surprised as everyone else and commented how Goku is incredible.
Yea, from my viewpoint. Trunks is still looking at Grade 3 as his power, it's a failed power, but in this situation with Goku, he's saying, holy shit, Goku has surpassed my useless power, by a wide margin. That's how I see it.