Your size is Medium

Your Dexterity score increases by 2, and your Intelligence or Wisdom score increases by 1.

Due to the ability to replace Kobolds within the trench coat, you can pretty much live as long as you want as long as you don't get all three killed.

Alignment is more what you would call "Guidelines" than actual rules. But let's face it, you're Three Kobolds in a Trench Coat, you're probably at least Chaotic.

Your base walking speed is 25 feet.

Accustomed to life underground, you have superior vision in dark and dim conditions. You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern color in darkness, only shades of gray.

Those silly pinkskins never think about second or third pairs of hands. As a result, you gain advantage on all Sleight of Hand checks you make from your unified form as even the most suspicious of shopkeepers don't think about watching your torso for more sticky fingers

In your unified, Trench Coated form, all three kobolds share the same, united, HP pool, Spell slots, and feats as if you were one creature. Spells like Barkskin, Longstider or Haste likewise affect all three of you like you were one person. Likewise, when unified, all three kobolds count as one creature when enemy spells attack you as well. Spells such as Charm Person affect all three of you equally, and damage taken comes out of the one health pool.

Since your three kobolds are constantly balancing on each other all the time, when you are struck by a critical hit or fail a Strength or Dexterity saving throw, you must make a DC 10 Acrobatics or Athletics (your choice) check to ensure that you stay together. If you fail this check, you fall to the ground Prone, and are forced into the Scatter! feature.

As a bonus action, all three kobolds exit the trench coat and run in opposite directions. The following properties apply:

Each kobold maintains any attributes, conditions, immunities, proficiencies, abilities, spells etc. that the unified form has.

Each kobold gains the Sunlight Sensitivity feature, normally waived while inside their protective Trench Coat

Each Kobold is Small and can move at 30 feet.

Your Kobolds all share the same Initiative, and you may choose in which order you wish to act each round.

Each kobold takes one third of the unified HP and temporary HP (rounded down).

Each Kobold has one attack per turn, and any Extra Attack features are lost

Each Kobold maintains the unified form's limitations such as spell-slots, feature or magic item cooldowns, and the like

Between the three kobolds, only one may cast spells per round.

Once per turn abilities can only be used once per round between the three kobolds

Each Kobold may take any number of items from your inventory when it emerges. If a Kobold takes an attuned item it counts and being attuned to that item, and if the Three Kobolds in a Trench Coat is reformed, the unified form regains attunement to the item.

During their turn the Kobolds may return to the coat and regain their unified form if they are all adjacent to one another. This does not cost an action, instead costing all movement for that round. The kobolds' current HP and temporary HP are added together to create the HP and temporary HP of the unified form. Any conditions or spell effects currently held by any of the kobolds are applied to the unified form. All equipment and attuned items are returned to the unified form. 

If another creature becomes suspicious of you, it may roll Insight against your passive Trench Coat Deception (8 + proficiency bonus + Charisma modifier). If they equal or exceed your score they realize you are wearing some sort of disguise. If they beat your score by 5 or more, they immediately realize you are Three Kobolds in a Trench Coat. If they roll less than your score, they are have no reason to doubt that you are in fact a human wearing a stylish trench coat and it's quite rude of anyone to suggest anything else, really. A creature can become suspicious again if you act strangely or fall Prone.

You can speak, read, and write Common, Draconic, and one additional language of your choice.