Does Assault count towards the Gust 3 Might limit?

Yes, Assault counts toward the 3 might limit for Gust.

According to RiftJudge FAQ #6637 and #1185, Assault is a passive ability that applies immediately when a unit is designated as an attacker during the cleanup phase before combat begins. Because Gust checks the unit's current might (which includes any active modifiers like Assault), a unit with a base might of 2 and Assault 3 will have 5 might and will not be a valid target for Gust.