I guess that the bonus is that if it fails, you still have a good copy of the data on the 36GB discs. If you choose to go ahead with this, schedule it for a weekend because the expansion/conversion process is likely to take a fair bit of time.
Migrate back to the 36GB discs one by one
Once complete, insert two 72GB discs and configure as a single 72GB RAID-1 container (or 144GB RAID-0 container if you must. Either will work, although RAID-1 is safer, especially as you may be running live during the expansion process )
Copy/image the RAID-5 container onto the new container.
Set the new container as active in Disk Administrator
Power down the server.
Remove (and carefully label) the RAID 5 discs. It is vitally important as the PERC 3/Di does not support drive roaming.
Insert the pair of 72GB discs into positions 0 and 1 (3 into 0 and 4 into 1)
Power up the server
When the PERC starts it will ask you to confirm the change of config. Enter to continue.
The server should boot. Log into the OS.
Insert the third 72GB disc into slot 2
Start Array Manager or FAST (whichever tool you are using)
Use the Reconfiguration Wizard (from memory it is the same name for FAST or Array Mangler) to convert from RAID-1 to RAID-5.
Wait. Wait some more. And wait some more. I'd expect this process to take some time.
When the process is complete reboot the server and run diskpart.exe so that the OS sees the bigger partition.