This may become an inscription written on a historical marker commemorating a mass grave. Which of the following is correct: Requiesce in Pace or Requiescant in Pace? The former was offered up by a young lady who was in my high school Latin class 50 years ago. The second was provided by translate.google.com. I think we are looking for third person, plural, subjunctive, present tense. I have seen google's form on a photograph of a slate grave marker dated 1725, but, the stone didn't have almost 300 years of patina on it.
Requiescant in pace is what you are looking for. Just as you say, third person, plural, subjunctive, present tense form, may they rest in peace.
(Requiesce in pace, instead, is singular, imperative mood, so the sentence is commanding someone to rest in peace, or figuratively, wishing it.)
Both the plural and singular (requiescat in pace) forms have traditionally been part of Christian prayers for the dead. Here a prominent example in Catholic liturgy, the Requiem, which is part of the Missa pro defunctis:
Requiem aeternam dona ei(eis), Domine, et lux perpetua luceat ei(eis). Requiescat(-ant) in pace. Amen.
Eternal rest grant unto him/her (them), O Lord, and let perpetual light shine upon him/her (them). May he/she (they) rest in peace. Amen.