Base Class: Rogue
There exists magic in this world beyond your greatest imagination, wizards and warlocks capable of both great and terrible things in equal measure - and then there are those who choose to do strange things, chaotic things for unknown reasons. It was a Wizard, they say, who granted the first Gnoll Rogues a gift in the form of protection - the ability to hide from a society who had come to hate their race. Some would call the magic odd, others would argue that it was a kind act; at the end of the day, it granted a new option for Gnolls seeking a new life: a form of brotherhood and trickery - allowing them to hide in the form of a Firbolg but only if there were three of them willing to do so.
And new talents come from every new discovery - the hidden gnolls were not any different.
Panic!
Starting at the 3rd level, you gain control of the magic to the extent that you can choose as a group to slip into your natural forms as an action. Your natural forms are equipped with what your combined form was, although you are all clothed in the same clothes that you were wearing when you were originally transformed. And much like the creatures that you fight against often do, the three of you share initiative and thus act on the same turn with all of your own movement speeds and independent actions / bonus actions. By the beginning of your next turn you have yourselves situated and ready to move. As a bonus action, you return to your combined form - and once you use this ability you may not use it again until after a short or long rest.
Unsettling Visage
At the 9th level, you gain the ability to partially transfigure bits of your figure to resemble your gnoll selves without completely separating in reaction to attacks. When a creature you can see makes an attack roll against you, you can use your reaction to impose disadvantage on the roll. You must use this feature before knowing whether the attack hits or misses.
Using this trait reveals your true nature to any creature within 30 feet that can see you. Once you use this trait, you can’t use it again until you finish a short or long rest.
Modern Literature
Beginning at the 13th level, you start implementing a plan that you fondly call 'Modern Literature' - also known as taking advantage of your grizzly forms to confuse and misdirect any creatures that may be attacking you. If a creature that didn't not see / notice you separating moves to attack you, you may feign an attack against one of your fellow adventurers and make a deception check with advantage to see if they believe you. Once you use this ability, it cannot be used again until after a short or long rest - and if one of the monsters sees you attacking on of its kind, you must make a deception check with disadvantage to see if the plan continues to work.
Triple Threat
Starting at 17th level, you master the art of separation - able now to emerge with weapons already in hand and with a thirst for blood sparkling in your eyes. As a bonus action, you may separate and immediately attack a creature - however, you may only use the attack action on the turn that you separate and not anything else. However, your attacks do function as they would if you were already separated and you get flanking bonus as you circle around the creature. Once you use use this ability, you may not use it again until after a long rest.