Yes I would consider banks to be under category 3. Hence, your job would be tolerable.
Why do I say tolerable?
First of all, banks are involved in both: halal and haram activities. Hence, by protecting the business, some scholars may deduct that you are indirectly assisting in sin.
Although, this does not mean that your income is haram. Your income is halal. However, as I mentioned earlier in other posts, a Muslim should always aim to have a career in a place free of any sort of haram activities.
Secondly, the security sector can be very unpredictable in the sense that, very quickly the security company could end up with different type of customers. As an employee, you have no choice over who to protect and not. Hence, to remove the hardship, I would say that it is tolerable to do such job.
A couple of points that need to be mentioned for others:
- the above ruling is specific to a security job.
- the ruling is regarding people employed by security companies who have different types of clients i.e. clients involved in halal and haram activities.
And Allah know best!