Here is how to make a cinematic screenshot (removes hero aura/stats bars etc as well). In this tutorial, i also set up the trigger for the animations without telling what the animations for different units are. Don’t have much time on my hand for aoe3 currently, but i will make a list of the animation for all units. Which unit do you want to behave like fighting? This also resolves the last point as the units won’t actually be fighting. Go further on if you want them to be really fighting →
Trigger for Age
New trigger → Run imediately / Active → Effects → Type → Player: set Age → Player 1-8 / Age 0-4 / Display Effect ON. You won’t see the effect on the editor but, if you followed my tutorial, you will see it in the game when playtesting.
Trigger Immortality (almost)
There isn’t something that makes your units immortal but you can use ModifyProtoUnit and make them have absurd amount of hp.
To do this, New trigger → Run imediately / Active → Effects → Type → ModifyProtunit .
The name of the units are strange in the editor, not like how they are in normal game. But there is an easy way for you to know them. Select the unit and then, top panel select Objects → Replace Unit. A side panel will come. Now select again the unit which name you want to know of. Done. ( If a @tantalus_dev_team read this, could you try to implement the same thing of Replace unit in the proto unit selection. Given the game already has the code to understand what unit it is that i am selecting, why the faq PROTOUNIT can’t do it)
As per this side screen, the name of the unit is ypDaimyoMototada. Now in the ProtoUnit box, search for that exact name. Select the PLAYER and what Field you want to modify, now it is Hitpoints, and then in the Delta (which is a weird multiplier that behaves differently for different fields) put a big number, like 10000… you get this as result:
10k hp Daimyo and when the trigger for the cinematic activates:
2 Units fighting for quite a long time without any UI.
Trigger Revolution (Just Flags)
( TBC… for real trigger)
Ah, you just want the flags? If that is the case, it is really simply. In that case just New trigger → Run imediately / Active → Effects → Type → Player:Override Civilization for Flag. Which will get you to this screen:
Then choose which player and what flag you want. As always, the editor won’t show the change but when you play test it:
Revolution Canada Flag !!! 
GL and HF in the editor.