Only that final category out of the 3 (believing that your dua won't be accepted unless you invoke the name of the Prophet Muhammad PBUH) you mentioned is actually considered Shirk in-and-of-itself by the overwhelming majority of Sunni Islam including the likes of Ibn Taymiyyah, Ibn Katheer, Ibn Al Qayyim etc.

The first 2 categories (which are asking anyone other than Allah, or using a Saint or a Prophet as an intercessor) are viewed as major sins which do not rise to the Shirk. This was - and still is - the position of most of Sunni Islam. Including the position of early Sunni Islam.

Because asking a dead person in their grave or a living person, cannot in and of itself be Shirk. To make it Shirk, there has to be other conditions present which indicate partnership with Allah SWT - such as asking with religious intent (Du'aa) rather than simply making a request (Istigaatha).

And there are other conditions as well which makes it Shirk, for example, such as asking for something only Allah can provide in the first place etc.

Just to be clear about the first category, using the name of the Prophet ﷺ to "increase" your chances of Du'aa being accepted is a type of Tawassul which was actually considered acceptable by many Sunni scholars including Imam Ahmad. But the correct view is that's it shouldn't be done because it's a religious innovation.

But if someone genuinely believes that Allah SWT only answers Du'aa after it has first gone through the Prophet ﷺ, then this indicates a type of partnership between Allah & the Prophet ﷺ. Ibn Katheer gave the analogy of a King who relies on his security guard to approach visitors. Since the King depends on the security guard, it's a type of partnership (even if there is a power imbalance). Hence by analogy, it must be Shirk if it's done with Allah SWT.