This is from dictation.
First of all, many people read this forum so recommendations given are sometimes interpreted to be the general procedures that should be used. However, in your case, this is not really a general procedure.
Here's the problem. You have a small partition that is unknown. The utility you are using, GPARTD, does a good job of finding partitions. In my opinion, you will have to delete that unknown partition before you can proceed to install Windows 10. With the program GPARTD you are given a chance to undo your operations before you hit the apply button. Once you have told the program to do something and you hit the apply button, there's no turning back. The contents of that unknown partition cannot be read, so there's no effective way to make a backup of whatever it is.
After getting rid of the unknown partition, use the same
GPARTD tool to resize the partition that is identified as dev/sda3. You can safetly reduce it to about 500 Mb.
Then after you have done that you can boot the Windows standard install disk and do a custom install. You will let Windows create a new partition and install Windows 10 on a new partition.
To recap, this no point in keeping that unknown partition because we don't know what's on it or why it's there. Because of its small size, it's unlikely that has anything important that you really need.
Hope this helps.
