What are three important characteristics of a periodic signal?

Answer 1

Period (or frequency), amplitude and phase. All periodic signals can be broken down into other signals... most commonly sine/cosine waves, but there are others too. These components will each have their own frequency, amplitude and phase that combine into the original signal.

The strange part of the question is the phase. A signal on it's own does not have a phase unless you provide some reference signal to compare it to. Generally, this comparison signal is implied by the context of your particular situation. When you decompose a periodic signal into components, however, it is almost always implied that the phase of each component is in reference to the fundamental component (So the fundamental has phase 0, while the others have phases referenced to that). This is done specifically so that each component will combine to create the original signal.