What are your thoughts on the vision that Bran has while being comatose about Sansa and Arya after the Trident incident surrounded by Hound, Jaime and Gregor ? I think it's after the Trident incident .
First up, it is definitely before the Trident incident.
Bran wakes up at the end of this chapter. At this point, Tyrion is still at the Wall where Jon whirls him around in joy after reading in a raven that his brother has woken up and will live. Tyrion leaves soon after and it is approximately three weeks later that he is back at Winterfell. And another four weeks or so, by my calculation, that he chances on Catelyn at the Inn at the Crossroads. The distance between KL and the Inn is universally described as about a fortnight, meaning - unless Catelyn hung around a while in KL after she speaks to Ned on the very day of his arrival - she gets to the Inn about a month after the Trident Incident. Tyrion could never have covered the distance between the Wall and the Crossroads Inn in the space of less than a month.
Consequently, Bran's dream happens weeks before the Trident Incident.
It's a prophetic dream, and probably has more than a little endgame foreshadowing in it, but just focusing on the passage you mean:
He looked south, and saw the great blue-green rush of the Trident. He saw his father pleading with the king, his face etched with grief. He saw Sansa crying herself to sleep at night, and he saw Arya watching in silence and holding her secrets hard in her heart. There were shadows all around them. One shadow was dark as ash, with the terrible face of a hound. Another was armored like the sun, golden and beautiful. Over them both loomed a giant in armor made of stone, but when he opened his visor, there was nothing inside but darkness and thick black blood. (AGOT, Bran III)
I can't help but think that there are more than the obvious conclusions to draw. It is, after all, shadows and not individual men that surround them all. The dream likely connects far into the future.
The hound obviously recalls Sandor, but also Ramsay, and hounds are generally described as opposed to wolves. Golden beauty recalls House Lannister, but the sun recalls House Martell, and even the golden company. The 'giant' is an image connected to Gregor, yes, but also to Tyrion and Littlefinger, and eventually probably Dany herself, who has seen herself behind Rhaegar's visor. What is an armor made of stone? A castle? An unfeeling heart? Black blood is connected to death and blood magic, but also the the Night's Watch.
Arya and Sansa both are connected to the image of giant-slaying, so I would just place this one on a shelf right next to the "prophecy" of the Ghost of High Heart.
It likely means more than one thing, but what exactly it is, only time will tell. :)