MELBOURNE, Australia - Red Bull have admitted being wary of a renewed challenge from rivals as Sebastian Vettel unleashes his bid for a historic hat trick of Formula 1 titles at the 2012 Australian GP.Team chief Christian Horner said Red Bull, which dominated the previous two seasons, may be disadvantaged by a key rule-change so was keen to find out the pecking order as the season starts in Melbourne.Horner said: "We've managed to tick most of the boxes. Of course you never know what your opponents are up to and there's quite a bit of gamesmanship that goes on at this time of year but we'll soon know the pecking order."JOINING LEGENDSPre-season testing in Europe, as well as exposing Ferrari's frailties, indicated greater competitiveness from McLaren, Renault and Mercedes, prompting observers to predict a much tighter season than 2011 when Vettel swept to 11 wins from 19 races plus 15 pole starts on his way to becoming F1's youngest double champion. Red Bull locked up the Drivers' and Constructors' titles with three races to go.Vettel stands on the threshold of joining the legendary Juan Manuel Fangio and Michael Schumacher as being only the third driver to win three or more championships in a row.Red Bull's success has been driven by technical innovation but along with other teams will have to operate without the pioneering, but now banned, "blown diffuser" which used exhaust fumes to create greater downforce.McLaren chief Martin Whitmarsh set the tone when he said that although his team, led by British ex-champions Jenson Button and Lewis Hamilton, did not look dominant, neither did any of the others. "The bad news is we don't look dominant, the good that nobody else does either. I sense we are competitive. The car seems quite reliable. The drivers like it and they seem to be quite buoyant."NO BITE ON BUTTONWhitmarsh added he was sure Hamilton would bounce back from a roller-coaster 2011 blighted by personal problems, including a split from his pop-star girlfriend. "He has exceptionally high standards and won three races in 2012 but that's not what he expects of himself. When you have a season that falls below your high expectations, I think you would, as a professional athlete, reconsider every aspect of your life and how you do that better."I think you can see that he's definitely focused on beating, one, his team mate, and, two, everybody else this time."Hamilton also refused to bite after Horner suggested that team mate Button, considered the slower of the two, would be Vettel's main rival this season."It's the first time I've heard of (Horner's remarks) but they definitely don't wind me up. There are a lot of opinions - everybody has a right to an opinion - but it doesn't really affect me. It doesn't really matter at the end of the day when I go out and do the job."The 2011 season is behind me and losing to Jenson (in the season standings) did not bother me. If it had been a case where we had been fiercely competitive throughout the year it would have been quite tough to deal with."DOWNBEAT NOTEHowever Ferrari has warned it might face further problems this season and left the final testing session in Barcelona tight-lipped after cancelling their drivers' media sessions.On arrival in Melbourne, which will host the season-opener on its inner-city Albert Park circuit, double champion Fernando Alonso struck a downbeat note for fans of the celebrated Italian marque."I think it is going to be difficult for us at this part of the season - we have a lot of work to do." Vettel, Button, Hamilton and Alonso are among six F1 champions lining up for 2012, along with Mercedes' Schumacher and Finland's Kimi Raikkonen, who is returning to the sport with Lotus-Renault.However Vettel's team mate, Mark Webber, who has added Australian markings to his helmet, says he is motivated for Melbourne after failing to finish higher than fifth at his home race. "There is something extra-special to win on your home soil but it is something that not a lot of drivers manage. I know I've got a good chance in our car so let's see what happens."