What is the RAID Solution on the server?

So I got a new gig supporting some older servers, NT4 as a matter of
fact. The customer had a disk crash in Oct and replaced it with a 36GB
drive. So here is what they have in the configuration and you tell me
your thoughts on best scenario for Disaster Recovery.

So right now there are 3 disks in the server. Two are 18gb and 1 is
36GB. The 18gbs are mirrored according to the Dell Array tool. Then
the 36GB is in a strip set? How is this possible? My feeling is they
are using Hardware RAID 3. Maybe. All the Data is stored on the 36GB
drive. The goal is to use RAID 5 so that we can HotSwap the
Replacement when needed. We have a 4th slot available my question is
do I add another 18gb by breaking the mirror with the OS or and
another 36gb and just create a mirror between the two?

What would you recommend? This is a PDC so its a big deal when it goes
down.

Cheers