You will probably not see a performance increase using 3 disks in a RAID 5 format...it actually may be slower than a single disk. However, what you with have is what RAID 5 provides -- fault tolerance.
Think of a single disk. Only pipe to pump data through.
Now think of how RAID 0 (striping) works. Now we have two or more drives to write to or read from...basically two or more pipes to pump data through...
Key: the more pipes the more data can be pumped through.
Now you have RAID 5 (striping w/ parity). With your three disks, you have two "pipes" to read or write through, but you also have to write parity to the third disk. This takes time and will lower any "gains" you might have gotten from writing (or reading from) the two disks.
So how do you increase performance? Remember the above key? That's right...add more disks. After adding "enough" disks, you will have enough pipes to pump data through to offset the time the system has to write to the parity disk.
There is a limit on how many disks you can use in a RAID 5 disk array before performance tops out. What that limit is I don't know...and your boss probably doesn't have the funds to find out (probably tens or hundreds of disks).
This pretty much sums my explanation. I would do a Google.com search on "RAID 5 performance" as do some reading from the Internet...the library at your fingertips...
later...