Transguild Courier.

[Munchlax]

Would I be able to run {Transguild Courier} in a three colour EDH deck?

[Remillo]

Nope.  It has a color identity of all Five colors, so it can only be run in a deck whose commander also has all five colors.

[Pleeb]

And just because I looked it up...

903.4.: The Commander variant uses color identity to determine what cards can be in a deck with a certain commander. The color identity of a card is the color or colors of any mana symbols in that card's mana cost or rules text, plus any colors defined by its characteristic-defining abilities (see rule 604.3) or color indicator (see rule 204).
Example: Bosh, Iron Golem is a legendary artifact creature with mana cost {8} and the ability "{3}{R}, Sacrifice an artifact: Bosh, Iron Golem deals damage equal to the sacrificed artifact's converted mana cost to target creature or player." Bosh's color identity is red.

[Munchlax]

Nope.  It has a color identity of all Five colors, so it can only be run in a deck whose commander also has all five colors.

Thanks. I thought it went by whatever colour symbol was on the card