On the 10th of May 2018, three prisoners, Kim Dong-chul, Tony Kim and Kim Hak-song, were released from North Korea.
Did anyone negotiate for the release or was it just a show of goodwill by Kim Jong-Un?
The way I see it KCNA said it was by the "suggestion" of Donald Trump (source) . Pompeo's meeting with Kim Jong Un and the context of the summit may have also been factors in making the decision (source). Ultimately, I'd say it was negotiated by Pompeo but Kim Jong Un had various reasons (like international image ahead of the summit) to make him lean on the pro-release side.