If I play Teemo Strategist from hand and deal 3 damage to Hwei and it resolves, then my opponent uses Flash to move his Hwei back to base, is his Hwei at 2 health or 5?

To answer your question, we first need to clarify a few things about game state and combat:

1. Damage Does Not Reduce Might: In Riftbound, units do not have "health." They have Might (their strength/durability). Damage is marked on a unit, and it persists until the unit is healed or the combat ends. If a unit has damage marked on it equal to or exceeding its Might, it is killed during the cleanup step.
2. Combat Damage vs. Non-Combat Damage: When you play a card like Teemo, Strategist to deal damage, that is damage dealt by an ability, not combat damage.

Regarding Hwei:

• Hwei’s Might is 5.
• After Teemo’s ability resolves and deals 3 damage to Hwei, Hwei remains a unit with 5 Might, but he now has 3 damage marked on him.

When your opponent uses Flash to move Hwei to the base:

• Hwei’s Might remains 5.
• The 3 damage marked on Hwei remains on him.

Because the damage is persistent, Hwei is still damaged. If Hwei were to take another 2 damage (totaling 5), he would be killed during the next cleanup step.

A Note on Timing and "Hidden" According to the current rules and the errata found in the RiftJudge FAQs (ID 6607 and 1353), Teemo, Strategist only triggers when defending. He does not trigger when played from hidden. If you played Teemo from hidden, his ability would not trigger to deal damage to Hwei at all, unless you were also declaring him as a defender in that same combat.