Question: my system doesn't have a hard disk, so I am using a USB stick of 16gb. I am running live ubuntu without installation, it's taking about 3 GB of space. is it possible that can I create a partition for USB so that I can save files in USB when using live ubuntu at the same time?

Answer: 
I also tried a lot to do this. Actually there is a term for this called "persistence". In my experience, it works only when I create the bootable usb drive with Rufus. In Rufus, when you are creating the bootable usb, there will be an option to set persistence and the size of the persistent partition. It will make the live boot slower but at least you will be able to save your changes.

(edit: Hi, I think I misinterpreted your question. If you just want to store some other files on the USB Drive, you can simply create a new folder in the USB Drive and store them, (you might not be able to use them while you are in live boot) but if you want to store files which you can access in the ubuntu during live boot and also want to save other software installed in ubuntu, then you should make persistent partition in Rufus as I told earlier )
Make Sure You *Activated* it Because it Came 0 Gb By default And 0 Means thats the **persistence** is *desactivated*.
There is it.

**strong text**[![enter image description here](https://i.stack.imgur.com/ol85k.png)](https://i.stack.imgur.com/ol85k.png)