Team principal Dave Brailsford said the three new faces reflected an emphasis on strengthening the line-up for the grand tours, particularly when it comes to the mountain stages, as well as nurturing British talent.
Team Principal Dave Brailsford said: "These signings will strengthen our squad even further for our second season on the road.
"Rigoberto and Xabier join from Caisse d'Epargne and both should come to the fore in the mountains in the grand tours, an area we have been keen to focus on.
Colombian Uran, 23, rode in this year's Giro d'Italia and recorded two top 10 finishes at the Vuelta a Espana, while Zandio has ridden the Tour de France five times and forged a reputation as a tireless domestique in the mountains.
"Rigoberto," Brailsford said, "is a super-talented rider, who has some great results to his name this year and we are delighted that he has committed his future to Team Sky.
"He's a young rider with fantastic prospects, exactly the sort of person we have been keen to get on board. He's an excellent climber who has already shone in the grand tour environment and I'm sure he will continue to go from strength to strength.
"Xabier is a rock-solid climber who brings us real experience; he is someone who will give his team-mates absolutely everything in the mountains on exactly the sort of stages which decide grand tours."
Dowsett's arrival will take the British contingent in Team Sky's squad up to nine, six of whom are aged 25 or under.
Brailsford added: "I'm delighted that Alex is joining us; he's a young rider who has come through the British system with flying colours and will fit perfectly into our set-up.
"We know just how talented Alex is and he's been showing that this year, winning the Under-23 individual time trial at the European Championships in Turkey this summer and getting some excellent results for the Trek-Livestrong U-23 squad.
"He's a real all-rounder with an incredibly bright future and we are looking forward to him developing with us at Team Sky and achieving his full potential.
"These three new additions sum up our squad perfectly - a blend of developing talent and grand tour experience with a strong British influence at the heart of it."