I guess this is sort of the reverse of this question: Change default location of programs / users
As always, the safest option would be to backup all your data before messing around with anything.
If 120gb is enough to store all your current data, from all three partitions, I would suggest first booting into recovery mode, and then moving everything to that partition. Then I believe you would have to edit fstab to be able to actually use the data.
Then, you can edit, i.e. merge, your partitions using gparted from a live USB stick, extending your 120gb partition and getting rid of the other two.
If 120gb isn't enough, I guess you would have to move things around a bit, for example backup to an external disk, and then merge your partitions before you can move the files to the single partition.
Since I'm not particularly well versed in the commands needed for the above steps, I would probably try the following. Perhaps not the most elegant, but I'm sure it's functional.
First backup all relevant data to an external disk, and then reinstall Ubuntu. Using the advanced settings from the CD or USB stick, merge the partitions and reinstall to that single partition. Then restore your backup data. I know that copying your /home folder also copies any configuration files (for example browser history). I've only done this with /home, but I would think that the same goes for the "/" and /media/data partitions, meaning you shouldn't lose any of your data. Just make sure that you backup all the hidden files and folders as well.