You can get access to all 3 armies at the same time (Yuri, Allies and Soviets) in skirmish, without having to capture an enemy base.
It does require you to play Yuri's Revenge and to have teammates (AI or players) that are playing as the allies and soviets.
When you start a skirmish match, start as Yuri and build up till you can make Yuri Prime.
Steps: deploy con-yard, build up Bio Reactor - Slave Miner - Barracks - War Factory - Psychic Radar and Battle Lab.
If you build a cloning vats, you get 2 Yuri Primes at the same time for the price of one.
Not move Yuri Prime to a friendly Con-yard and hold ''CTRL'' and attack the con-yard, this will turn it into a building owned by you.
Now you have access to that factions tech, I would recommend quickly building a war factory and repair depot, that will allow you to build a con-yard of your own of that particular faction.
To give the building back to its rightful owner, you will or have to mind control a unit, building or kill Yuri Prime, can be from a friendly or enemy.
If you do it to both a Ally base and a Soviet base, you will have access to the full roster.

Or you can of course keep picking up crates and hope for the best to get the other con-yards.

And now that you know how to ''Borrow'' your friends buildings, what is stopping you from selling their full bases?!

https://sp.zhabite.com/sharedfiles/filedetails/?id=3498442654