Line 1
Line 2
Line 3

Line 1 Line 2 Line 3 

I see you typed this text in "HTML text" mode of editor.

Why don't you type the following?

Line 1<br />
Line 2<br />
Line 3<br />

<pre> tag is not currently supported properly.

Yes I can, but there can be big chunk of code. I can also make a program that can put <br/> at the end of each line. But I can, not all users/clients can or want to put their time in it.

pre tag suppose to keep the actual format.

It works good, If I do not press 'Design' mode button.

Indeed, "Design" mode is the main mode of Editor.
Users should type text, style it, insert links and images in this mode.
"HTML text" mode is mostly for viewing the result HTML code.

If you type in "Design" mode:

line1
line2
line3

Than you can see in "HTML text" mode :

line1<br />
line2<br />
line3

Yes, you're right teo. But that thing is buggy. I've html code so it will be a bit complicated to first paste all html code then change pre tag code in design mode.