This would be a lot easier to explain with a diagram, but there are no drawing tools in this app, so.......
Imagine the letter "V". It's basically an angle. Just one. The vertex is the point of the "V".
Now imagine the letter "X". It consists of four adjacent angles (not to mention the larger angles created by adding the adjacent angles together) with a common vertex. The common vertex is where the two lines forming the "X" intersect.
Imagine a math diagram that looks like a "V", and the vertex has a letter name, say "A". If I talk about "angle A" it's clear which angle I'm talking about. There's only one angle in the diagram.
Imagine now a math diagram that looks like an "X", and the intersection point of the two segments that make the "X", the common vertex of the angles, has a letter name, say "B". If I talk about "angle B", it's unclear which of the four angles (or the larger angles consisting of two or more adjacent angles) I'm talking about, since multiple angles have the same vertex. I would need more information to name a particular angle, namely two endpoints of two of the segments making the "X".
To sum up, you can name an angle with one letter when there is only one angle that has the vertex named by the letter.
When more than one angle shares a vertex, you have to use three letters, one on each ray or segment of the angle you're interested in, with the vertex in the middle.