How to enlarge a Linux partition by shrinking another?

Question

linux

Hello.
There is a very important question, I sincerely hope for your help.
What is the situation:
I have Elementary OS and Windows 8.1, Windows is installed on the sda1 partition, it is also a boot for Linux and it seems to be Windows, Linux on sda3, sda2 is the partition from which you want to separate 100 GB and assign them to sda3 .
GParted

Standard utility

When I start the PC, the Windows bootloader appears, in which I select Elementary and then GRUB is loaded, where I select Elementary and enter it.
What I do:
1. I release 100 GB from sda2, they are released.

2. I increase sda3 by 100 GB. // I can’t do this.

But I can’t do point 2. the system does not allow it.
I tried all this with Windows, I did everything successfully, but Linux refused to boot, and I had to reinstall it.
Help me please. Because it's important.

Answer 1

But I can’t complete point 2. the system does not allow it.

First you need to unmount. Try the same steps by booting from a live distribution with gparted.