The tie-up coincides with the latest release of MSN Messenger and will see users offered the Critters in a variety of forms via Messenger including wallpaper, emoticons and animated winks, which users can send to each other.
The Critters, which include a panda, turtle and monkey, have been used by the Hutchison-owned network in its press, radio, TV, online and outdoor advertising over the past year. The characters were invented to appeal to 3's target audience of young people.
The company is aiming to establish cult status for the Critters by tapping into MSN Messenger's 8.5m user base, having already offered them for download via its website. More than 10,000 users have downloaded them so far.
3 chief operating officer Gareth Jones said that the MSN deal offered the ideal platform for the mobile operator to represent its brand values of being "playful and a little bit cheeky".
The Critters have been adapted by Meme Digital, with MindShare and mOne brokering the deal.
3 is the fastest-growing 3G network and has more than 3m customers nationwide.
If you have an opinion on this or any other issue raised on Brand Republic, join the debate in the .