United as Three

Requires three people of the same faction.

Buy all three items on the auction house or craft them yourself. (Battle Flag: Phalanx Defense, Battle Flag: Rallying Swiftness, Battle Flag: Spirit of Freedom)
Nobody needs to be a tailor.
Take the portal to Tiragarde Sound in Valdrakken. (Must be BFA zone to work)
Give one item to each person. Swiftness must be used by a mounted player. The other two are unmounted.
Do a countdown and use them all at the same time.

Achievement earned.

Warning: there is a bug currently where the player who used the Swiftness flag does not receive the achievement, so you will have to do it again 5 minutes later for that person. We just had the player who'd originally used Rallying Swiftness instead using Phalanx Defense while a different player used Rallying Swiftness for them, and they got the achievement as well on the second go.

Just got this achieve without ever using the Swiftness flag. It popped just by using the Freedom and Defense flags by 2 characters. No need for a 3rd.

You can actually do this solo. Might be hard without a second monitor and good PC though.

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• Get the three banners on three different characters and assign them to a hotkey. Park them right next to each other somewhere in BFA.
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• Open Battle.Net and click on Play WoW to open a second instance. As soon as WoW opens and it says "Connecting..." press one of the 30s banners.
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• You will be disconnected in your first WoW instance. Now you need to be fast - login to your second character. As soon as the loading bar is full, open a third instance of WoW. Then go back to your second and spam the hotkey.
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• Now quickly login to your third character and spam your hotkey to place the last banner.
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This worked for me on my first try. The whole process took around 27 seconds, so you will need to time the disconnects properly. I know it sounds complicated, but at least you don't have to find other people and do the whole thing three times because of the bug. Also, be careful not to park the characters in Zuldazar harbor if you haven't done any quests, since you will be in different phases. I just flew all of them to a nearby island.