What you describe is a political issue, not a technological one. Unless you've already made strong friends with the IT staff, and _earned_ their respect and trust, expect them to resist you at every step. In particular, expect their middle management layers to disagree with every move and sabotage them behind your back. And I'm afraid you're starting out with a basic confusion: "3000 employees" is not a medium sized, company, it's a _big_ company. I'm also afraid that if you think of it as a medium size company, you'll be tempted to micromanage individual employees and pet projects. You're not going to have time for this, really.
Earning the respect and trust of the existing staff is going to be awkward. You're going to need that: they're going to need questions answered, clear decisions made with clear justification, and a clear set of technology and social practices so they know what is expected. Documentation, backup, and standard practices for network and environment configuration are all going to come from or be signed off on by you. If they disagree with a practice, they need to be safe to express it safely, work with you to iron out the differences, and if it turns out one of you were dead wrong, to get the other's insight credited and the other person educated on why the right practice is just that.
And I'm afraid many practices aren't that clear-cut: flexibility versus recoverability, reliability versus expense, growth over repair are all going to require decisions coming from you, as an architect. And even when you make the "right" decision, sometimes things are going to break that the "wrong" decision would have avoided. Some of them are going to be _large_.
That's partly why the existing staff are afraid of handing over admin access. Another possibility is that they're _embarrassed_ and you'll have immediate visibility into what they're embarrassed about. I've had tremendous difficulty with staff leaving in unannounced backdoors "to get their jobs done" and not wanting me to know about them. I've even been booted right off a project because I kept going to the manager with "your technical leader is putting in backdoors for his telecommuting here, here, and here, in direct violation of SOX guidelines: I can't sign off on this".
Anecdotes aside, you're going to have to establish trust with these people. From your short description, it sounds like they've been thinking of you as the enemy. If they're unhappy and can't work with your vision, check if your vision is confused and you can accommodate their ideals and goals. If you can't incorporate their goals, or if they're just plain wrong, show them the door gracefully. And let them know what your "vision" is, so they can be sure if you'll like their plans or if they're going to have trouble convincing you something is workable.
The best way to show them the door is to help them get hired somewhere else better suited to their neds as well as yours. I've had good success with staff who'd outgrown our environments, or our environments had moved to new practices they didn't care for, helping them find work with partners and other companies who appreciated their approach. I've even done a few flat-out exchanges: "You need a new security expert, we need a new backup expert", we've swapped staff, and we both came out better for it.