If you look closely, you can see that the installer is warning you that you are loading from the hard disk partition sda3 and you will not be able to install it into the same.
From the documentation of unetbootin
If you provide a LiveCD iso file, such as the Ubuntu desktop iso, to UNetbootin and use the hard disk install mode, the resulting install will NOT be a full, standard hard drive installation. Rather, you are simply booting into the live environment, the same way as if you had booted from a live CD or a live USB, except it's being loaded from your hard disk instead. This is often referred to as a "frugal install", as it doesn't install a dedicated bootloader and doesn't make a separate partition, but rather piggybacks off the existing OS's bootloader and installs the files for the live environment inside the existing partition.
You have to use a USB drive and make it bootable using unetbootin, and then boot from that USB drive and install the OS onto your hard disk.