If a card kills a unit with 3 might and the unit has 3 might and a buff (making it effectively +4), is it a valid target?

Whether the unit is a valid target depends entirely on the specific targeting requirement of the card you are playing.

In Riftbound, a unit's current Might is its base Might plus any active modifiers, including Buffs (Rule 703). If a unit has a base Might of 3 and a Buff, its current Might is 4.

• If the card says "Kill a unit with 3 or less Might": The unit is not a valid target because its current Might is 4.
• If the card says "Kill a unit with 4 or less Might": The unit is a valid target because its current Might is 4.
• If the card says "Kill a unit": The unit is a valid target regardless of its Might.

According to Rule 703, each Buff contributes +1 Might to a unit. When determining if a unit meets a targeting requirement based on Might, the game looks at the unit's total current Might at the time the card is finalized on the chain (Rule 355.11).