I disagree. There are no hard and fast 3 top traits. It depends on the organization, staffing, and your position within it.
If I had to pick one that is critical to any good maintenance engineer, it would be taking care finances. You can be the best troubleshooter, personnel manager, and steward, but if you overspend(without good cause), your peers WILL replace you. Money talks!!! remember this from someone who has learned it the hard way.
Good personnel management applies not only to maintenance but all areas so I will not list it as a trait as it is a given.
#2(I agree with Victor). Know your equipment. Learn it inside and out. Study maintenance principles inlcuind TPM, Preventive and Predictive as well as process control programs such as 6-sigma, lean manufacturing, Kanban etc. DO NOT think that one system alone is your end all be all. It will usually be a combination of techniques and processes that help you maintain equipment and PRVENT REOCCURENCE FAILURES!!
#3 Know your role. If you are the troubleshooter, do the job. If you are the supervisor do NOT troubleshoot unless required(and you will know when this time comes). You get involved in individual breakdowns and you lose focus on the bigger picture. This will hurt your facility and the way your peers see you.