Why would Italy ally itself with Austria (it wasn't Austria-Hungary until it lost the Austro-Prussian War) ? Austria still controlled Venetia and Italy definently wanted that. Unless Austria agreed to hand over Venetia to Italy, the best U could hope for is Italy remaining neutral and even then it would want something in exchange, especially if Prussia is promising Italy Venetia. Also why would France from an alliance with Austria? If France's mind Austria would have been a b0gger threat then Prussia.
Italy controlled the majority of Venetia at this time, although Italy still desired Istria and Trento from Austria. This, along with Italy's irredentist claims on France, make me very skeptical as to whether Italy would ally herself with France and Austria. In my opinion, she would be more likely to declare neutrality, throwing herself into the fight as soon as Prussia promised enough (and was doing well enough.
Also, France did not see Austria as a larger threat. In fact, there were many Frenchman (including, arguably, Napoleon III himself) who thought Austria would make a great ally against Prussia. If France put more effort into courting Austria as an ally, it is possible to get both of them to take on Prussia.
The question is, could they win against Prussia? Prussia would have to split her forces, but if she goes against Austria first, the France she would leave in her rear would not be particularly aggressive. The French did launch a limited offensive in the Saar at the beginning of the OTL war, but the lack of offensive spirit among her troops, despite their greatly superior small arms, soon showed. I don't see an Austrian alliance alone changing the mood of France's troops. Unfortunately, I know less about the Austrian Army of the time, so I would have to defer to a more knowledgeable forum member on the subject of their preparedness for war.