Present Perfect: States a certain time or event has happened in the past and the time or event is ending in the present.
Past Perfect: States that a certain time or event has happened in the past and has finished in the past.
Future Perfect: States that a certain time or event will happen in the future and will finish in the future.
They have already slept.
She had already slept when they arrived.
He will already have slept when they arrive.
They finished sleeping directly before the current time. The exact time may be unknown, but this is not important.
First, she finished sleeping. After she slept they then arrived.
First she will finish sleeping. After she has slept, she will then arrive. She will finish sleeping before they arrive.