Confusion about timing

[Kerobelis]

Hi, a quick question about timing, or more specifically, when does a spell end?

This happened in our last game. Creat pit was cast on the BBEG by a 3rd level caster. He moved up and cast the spell. The BBEG failed the save and in the pit she went. The duration is 3 rounds. When does the spell end?

At the start of the wizards 4th turn?
At the end of the wizards 4th turn?

Some other time?

[King of Vrock]

If a 1 round /caster level spell is cast in round 1 the CL 3 spell ends just before the wizards turn in round 4.

--School of Vrock