The reason on why this is happening is because in Disk Utility, once a partition has been created it can only be either split, or reduced in size. That's why you were able to initially split your MacBook Pro partition in three different partitions.
Unfortunately, your only course of action is to boot inside your OS X Installer and from there re-map your hard drive. Consequently, this means that you'll have to format your hard drive and reinstall OS X and then restore your data from your hard drive back up.
If you ever decide to go ahead and format your hard drive I strongly recommend having some form of back up for your data. Things like uploading your personal files to the cloud or USB thumb or external drive are a good start and having copies of the third-party application installers can certainly help in the proccess as well. The ideal and less complicated route is to just attach an external drive and back up all your files via Time Machine.
I hope this information brings some clarification to your issue.
Yum3ji wrote:
Last year I had to create three partitions (120 GB for Macintosh HD, 510 GB for Data, and 120GB for Spare) on my HDD for a program I was taking. However, I'm finding that a 120 GB partition for Macintosh HD isn't sufficient for my needs anymore and would like to know if there is a way to expand the allocated size. Selecting the Macintosh HD partition and resizing the bottom-right corner will only allow me to decrease the size of the partition, not increase it. I've also tried doing this booting from a different partition and USB thumb drive.
