The ACO releases a list of drivers eligible to take part in the race each year. The listed drivers all have previous Le Mans experience.
The 2006 list is here.
The regulations for drivers not on the list are as follows:
6.1.5 – The list of the drivers automatically admitted to take part in the
test day and the qualifying practice sessions is published in the
appendix.
The drivers not included on this list are required to take part in the
test day, and cover on this occasion a minimum of 10 (ten) laps of
the racetrack, in order to be allowed to participate in the qualifying
practice sessions.
An allowance is made for listed drivers unable to attend the Test Day:
6.1.4 – At least two drivers of the final race team for the qualifying
sessions and the race must be part of the team registered for the
car on the occasion of the test day. If this is not the case, the team will
not be made with more than two drivers.
So only two driver nominations need to be made by the Test Day. The third driver must be registered by 13th June.
Hope that goes some way to answering your question.