Twobox Toolkit.

It has an option to relay all whispers or guild chat or raid/party chat to either the main in a tell or in a chat channel.

All three of my characters are in the same guild, but i use it for tells. Basically, my characters are named Ryanone, Ryantwo, and Ryanthree, and they are all in a channel named Ryan. Ryantwo and Three don't have the channel displayed though. And in that channel they repeat any whispers that they get sent. Very convenient.