It helps to think of this scenario as a deathmatch game and act fast rather than building up your base and economy.
I am trying to follow this strat:
but Romans either get a castle or cavaliers by the time my knight ball reaches their base. However sniping the Alan's (teal) TC and few military buildings is the same. Be sure to save wood for a blacksmith and a market to upgrade and trade for resources for your knight ball. For the Visigoths (green) send some of your knight ball over with your remaining tarkans to kill them off, try to be fast with the Alans so you can reach green before they get a castle up and spam huskarls. Be careful as green mainly train pikes and longswords. Like in the video you can lure their army away or harass vils building castles with your scout and a few knights as you are building up your force.
You want to kill or cripple the Visigoths and Romans before they get a castle up or upgrade their units. Visigoths start in castle age while the Romans start in imperial. If blue upgrades to cavaliers unless you strongly outnumber them it's probably gg.
Alternatively, send your scout to the roman base, attack a TC and lure their army towards your ally. Rally about half your stables outside blue and the other towards the green base. After killing the Alans, send your tarkans in the direction of the Romans and your 1st knight ball and snipe their 2nd TC in the north, castle foundations and builders before the Romans can get a castle up. It's important to keep LOS with a knight in their base to find these.
While this is happening keep half your stables piling knights towards the Visigoths, raiding their eco and preventing them from castling up their base.