1) Period - A length or portion of time. 2) Stage - A point, period, or step in a process or development. 3) Phase - A distinct period or stage in a process of change or forming part of something's development. The only significant difference I see is that a period is strictly a span of time (ex. the Jurassic period); a stage is also a span of time, but it has a nuance of development (ex. the adolescent stage in life); and a phase is a span of time associated with change (ex. the fluid state of matter). Don't worry about these difference so much though. You'll get the hang of it after enough practice!

Thank you, kaka. But,, it is kind of an academic explanation. I still cannot understand how to use them.

For me, i think that Stage refers to a particular level that you are at, and Period is more like a span of time, and Phase is like a state of transition (it is like something just passing by) thats what i think, haters gon' hate! but thats my 2 cents. Ciao!