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Is this possible? I've been told that it is only 1 allied detachment per FOC so only in games above 2000 points you are allowed to do this.

But if I take only formations from three different codexes would it still be bound? I'm trying to get a Harlequin Dark Eldar Craftworld Eldar army going.

Whoever told you that is either living in 6th edition, or is referring to specific tournament rules.
In 7th, you can take any number of detachments, in any combination, with only one or two limitations (which are almost entirely only about the Allied Detachment found on page 122 of the rulebook).
An army is Battleforged (i.e. Bound) if every model is in a detachment (formations are still detachments).

You could, for example, have any of the following as a legal, Battleforged, army:

-Eldar Craftworlds Warhost detachment (Guardian Battlehost and an Aspect Host), Harlequins The Heroes' Path formation and a Dark Eldar Allied Detachment (Warlord cannot go here).

-Harlequins Masque detachment, Craftworlds Allied Detachment, Dark Eldar Allied Detachment (Warlord must be in Masque detachment).

-Craftworlds Seer Council formation, Harlequins Cast of Players formation, Dark Eldar Realspace Raiders detachment.

-Craftworlds Combined Arms Detachment, Inquisition detachment (1 Inquisitor), Chaos Daemons Combined Arms Detachment, Tyranids Allied Detachment.