The dragon briefly shown in the first episode of House of the Dragon season 3 is Tessarion , the blue dragon bonded to Prince Daeron Targaryen.

Why this matters

That glimpse is meant to signal Daeron’s arrival in the wider conflict, and Tessarion is one of the key dragons on the Green side.

Episode context

The season 3 premiere centers on the Battle of the Gullet and is packed with dragon combat, which is why that blue dragon stands out so much.

Tiny spoiler note

If you only noticed a quick blue dragon shot, that’s the one most recaps are identifying as Tessarion.

TL;DR: it’s Tessarion.