If Yasuo Remorseful conquers with his when I attack at the Trapping Grounds battlefield and has an excess of 3 damage with his ability, do you get to play the bird token?

No, you do not get the Bird token.

Here's why:

This scenario has two problems working against you.

First, no Combat Damage Step occurs. When Yasuo's attack trigger resolves and kills the defender, FAQ #4462 confirms that cleanup kills the defender and no combat follows — there's no defender left. Since no Combat Damage Step happens, no damage was ever assigned during combat. Trapping Grounds specifically checks whether you assigned 3+ excess damage — and assignment is a combat-specific action (Rule 460.2.c). No combat step = no assignment.

Second, even if a Combat Damage Step had occurred, the damage from Yasuo's triggered ability isn't "assigned" damage anyway. Per FAQ #7566 (Tryndamere ruling), only combat damage assigned during the combat damage step counts toward excess/exceeding damage requirements. Yasuo's trigger deals damage directly via a triggered ability — it's not combat damage assignment.

Yes, Yasuo does conquer — after the Combat Showdown resolves with no defender, the Resolution Step establishes control and you Conquer. So Trapping Grounds will trigger. But the conditional ("if you assigned 3 or more excess damage") won't be met, and the Bird token won't be created.