So, you are tired of the game on one monitor, and are willing to spend a few hours on the interface fit for themselves.
So, open stellaris folder and go to \interface subfolder
All UI elements coordinates contained in .gui files

for example open main.gui
we see x and y coordinates
for containers, bars and buttons.

container is entire window with buttons, layouts and lists.
we need x coordinate for align window
for example find:

buttonType = {
name = "button_topbar_empire"
position = { x=60 y=-7 }
quadTextureSprite ="GFX_topbar_empire_button"

where x=60 our position on screen
add you width to this, in my example - 1920

result:
buttonType = {
name = "button_topbar_empire"
position = { x=1980 y=-7 }
quadTextureSprite ="GFX_topbar_empire_button"

you can do it for all main UI elements

Second important panel in stellaris - outliner.
In default is pin and not movable.

go to outliner.gui
first what we see:
guiTypes = {
containerWindowType = {
name = "outliner_toggle_window"
size = { width = 32 height = 32 }
position = { x = 200 y = 120 }
show_position = { x = -65 y = 105 }
hide_position = { x = 200 y = 105 }
animation_time = 500
animation_type = decelerated

orientation = upper_right
moveable = no

for moving outliner change yes for moveable (it worked another UI elements too)
it for small control elements
add our with for x coordinates again

we change position to center monitor, but it continue hide animation to side

for hide coordinates change hide_position

himself outliner begin
containerWindowType = {
name = "outliner_window"

again add to x coordinate your width resolution for align to center monitor
u can modify hide coordinates too

Also outliner have tooltip parameter
positionType = {
name = "outliner_tooltip_offset"
i set up 300 and 5 for me

It is most harder file for modify, contain most subincluded containers with UI elements. Hard for searching.

for planet view frame responsible
containerWindowType = {
name = "planet_view"

and "planet_view_panel"

do not change coordinates for subcontainer UI elements, it broke GUI.

Each panel has a parameter for hiding and speed animation
Example:
show_position = { x = -2180 y = 50 }
hide_position = { x = -1840 y = 50 }
animation_time = 500
animation_type = decelerated


Conveniently will set up coordinates for Empire, Contacts, Ship Designer, Situation log and Technology hide to upper edge screen side
for Fleet - lower edge, etc

You can change also speed animation for this, faster or smoothing.

Yes, notifications store in \common\defines subfolder
file 00_defines
open it and search

NOTIFICATION_MESSAGE_XPOS

i set up 2100

also u can change FOV for triple monitors

it in begin
FOV

i set up 60 for me.

So, you spend entire day, editing almost all files in interface, set up all parameters.
Now go to play! :)