If you had a bit more time, I'd suggest that you contact each of the participants and ask them a few questions. It may be too late for that, but they will probably want to know the order of presentation.
They will certainly want to know how much time they each have and how you should (or should not) signal them as the end of their slot approaches.
You need to know how questions from the audience are handled at the conference - after each presentation - all together at the end - not at all. Make sure that there is time for questions, if allowed.
If someone ends early, it is usually appropriate to delay the start of the next speaker so that those people who want to hear only part of the panel have time to arrive and/or leave without disruption. But that may vary with local custom.
If possible, try to meet the presenters before the panel session.
Coordinate with any volunteers that are available for such things as water or special needs.
Get there early to make sure the setup is correct.