If Glasc Mixologist dies at the same time as a 3 might unit, can I play that 3 might unit?

Yes, you can play that 3-might unit.

According to FAQ #9392, when multiple units die simultaneously (such as Glasc Mixologist and another unit), their Deathknell triggers are added to the chain as pending items before the units are moved to the trash. Because the game "remembers" them in the trash once the triggers resolve, you can target the unit that just died with the Deathknell effect of Glasc Mixologist.

Sequence of Events

1. Simultaneous Death: Glasc Mixologist and the 3-might unit are killed simultaneously.
2. Deathknell Triggers: Both units have Deathknell abilities. These triggers are added to the chain as pending items before the units are moved to the trash (Rule 734.1.d.2).
3. Chain Resolution: You are the controller of the triggers. You choose the order in which to add them to the chain (Rule 808.2.a).
4. Resolving Glasc Mixologist: When you resolve the Deathknell trigger from Glasc Mixologist, you are instructed to play a unit from your trash with an energy cost of 3 or less.
5. Playing the Unit: Because the 3-might unit is in your trash at this moment, it is a legal target for the Deathknell effect. You may play it from your trash to the battlefield, ignoring its energy cost.

Note: This is based on the mechanics of Deathknell and simultaneous triggers as described in the provided FAQs. No official rule chunk explicitly details the "simultaneous death" interaction, but it is consistent with the established Riftbound FAQ rulings.