Here's what happens.
Super Buff Trunks > Perfect Cell >> Buff Vegeta >= Buff Trunks
We realize that Super Buff Trunks is stronger, but his power is useless because of his speed. Cell tells them all to train and come back stronger so he can have a challenge.
As soon as Goku comes onto the scene, he checks out Cell. He is able to pick up on Cell's dormant power. Cell is way, way stronger than what he is showing, and it surprises Goku.
Goku comes back and tells everyone the news. Now the new mentality heading into the Cell Games is this...
Real Cell >>> Cell they fought
So everyone is completely flustered.
Goku does his power up...
50% Goku >> Cell they fought
And now because Goku doesn't think he can beat Real Cell, they assume Cell is probably somewhere around 50% Goku's power...
Piccolo trains long and hard and brings himself to his potential's full extent to more or less be on par with the power of Goku. However, Piccolo is clever enough to know that what Goku has shown, isn't all that he has. He tells Goku, as I am now, I still have no chance correct? Goku has a much better sense of Cell's power then Piccolo does, and Goku flat out tells him he is shit.
This is alarming to Trunks as he thought 50% Goku's power had to be topping out somewhere around Cell's. But lo and behold, Cell and Goku are both way, way above what he thought they were.
Vegeta either hears about it from Trunks, the group talking or he makes up his own mind about Kakkarot, knowing him so well.
Piccolo and Vegeta most likely come to their own conclusions, but they still have some hope they are wrong, a slight hope, but hope nonetheless. Trunks knows his power is garbage, he has no confidence whatsoever in his ability to do anything.
The ROSAT is a massive, massive power up. Far more powerful than most think.
Vegeta, PIccolo, Trunks all match Goku's previous power or surpass it, but only marginally so.
Piccolo, Vegeta and Trunks are all garbage compared to Goku, Goku most likely can easily handle all three of them at the same time.