If the battle of the Citadel happened, and the Alliance did not save the council, then the turians lost too many assets to go full on offensive early in the conflict, so they might have to play defensive for a time, until they can replace the lost ships and soldiers.
According to ME2, the turians are no longer able to effectively patrol the Citadel space, so a lot of these patrols and duties have been taken over by the Alliance. The turians though still have the most military assets among all the races, and if they no longer have to patrol in Citadel space, their remaining fleet very well could be enough to defend turian space until more ships can be built and more troops can be recurited.
The turians have the advantage of numbers in trained military forces, as every adult turian has completed their military training and they can be conscripted and sent to battle in short notice, whereas the precentage of soldiers among the humans is really small, so conscripting, training and equipping new troops would cost more time and resources. I think this could be the decisive factor in the war, since most of the technology used by two races is equally advanced, but the turians have the advantage of numbers AND trained troops (so it is not even the untrained, badly equipped but numerous Red Army vs. the highly trained, but smaller Wehrmacht with better equipment).
The volus wouldn't be much help for the turians in the field of battle, but their financial support might come in handy in equipping and maintaining the war assets, and the burden of the war economy would be shared between the two allies.
Cerberus could go full black ops on the turians and do some damage with sabotage behind enemy lines on some colony worlds and on less significant warships, but I don't think they could do much damage to the main fleet or to Palaven, as probably any human interest in turian space would be shut down once the war breaks out (so no corporations as Cerberus fronts) and any humans would be detained/deported (so no human agents).
Humanity's only chance would be winning the krogan to their cause, as they hate the turians with passion and are strong and resilient warriors, and despite the genophage, they still posess the numbers to provide a significant threat, especially if united under a single banner.