It’s best days may be behind it in online search, but Yahoo (NSDQ: YHOO) is proving it’s still a contender in mobile. The search engine and portal struck an ad deal today with mobile carrier 3 UK, in which it will sell the carrier’s banners and sponsored links inventory on the 3 UK portal, Planet 3. Previously, 3 handled their inventory in-house. Portal Advertising head Neil Andrews said that they chose Yahoo for its wide experience. In February, T-Mobile chose Yahoo for a similar deal, dumping its previous partner Google (NSDQ: GOOG). Yahoo’s partnership with Three means that it now has access to three of the UK’s 5 mobile operators. Vodafone (NYSE: VOD) has used Yahoo for several years now.