MTP protocol on Ubuntu 10.04 is very unstable. Fortunately there is a FUSE filesystem called Go-mtpfs, created by a Google employee because "mtpfs was very unstable".
To install go-mtpfs on Ubuntu and connect Samsung Galaxy S3 using MTP on Ubuntu just do the following:
sudo add-apt-repository ppa:webupd8team/unstable
sudo apt-get update
sudo apt-get install go-mtpfs
sudo apt-get install go-mtpfs-unity
Then from "Dash home" search for "mtp" and an application (actually a script) called "Mount Android Device" should come up.
Right-click and choose "Mount Android Device".
The phone will be mounted as "MyAndroid" device.
Why did I choose go-mtpfs in the first place?
mtpfs gave me an errors and loaded for long minutes or not at all.