There is no way to know the answer to this. We cannot really pull the three apart and experiment to see which was more important. There is clearly a case to be made for each of them. However, I would tend to argue that “steel,” defined as non-military technology, was the most important.
Of course, guns and germs were important as well. The germs were hugely important in that they killed off huge numbers of the people whom the Europeans conquered. When you have a factor that kills more than half of your enemies, it is clearly important. Guns are similarly important. It is clearly the case that the Europeans would not have had such a relatively easy time pushing the Native Americans and the Aboriginal Australians off their land without guns.
However, I believe that technology in general was the most important of all. Without things like ships, the Europeans would never have gotten into position to deploy their guns and to, inadvertently, spread their germs. Without technology, they would not have been able to exploit resources so efficiently in the lands that they conquered. Without technology, I would argue, the guns and the germs would have mattered much less than they did.