According to Dragon Ball Super's manga, Trunks is able to transform to SSJ2 which was used to defeat Dabura and Babidi. When he traveled to the past, he didn't know about Goku's SSJ3.
In the fight against Zamasu and Black, he transformed into a powerful Super Saiyan form, for a moment and could handle a battle with Zamasu, apparently this is not a stronger transformation like SSJGSSJ (Super Saiyan Blue).
Also this transformation has a similar Ki aura of a Super Saiyan Blue, but he keeps hair of a SSJ2. So, what transformation is this?