I am an Indian, and English is my second language. Inspite of tossing a few odd ones every now and then in my writing as well as in speech, I must admit I sometimes do wonder about the difference. This is like one of those obvious amenities, taken for granted in any sphere of knowledge to be implicitly understood, those that we use overly, but do not so fully claim knowing over their technicalities. So I ask, what's the difference between idioms, proverbs and metaphors?
A proverb, unlike idioms, is almost always a complete sentence, e.g. "A friend in need is a friend indeded". Metaphores are more complicated. It could be a phrase (like idioms) or it could be an entire paragraph (metaphoric language).

That's only one cosmetic difference, from the top of my head. I'm sure our distinguished contributors will elaborate at length on the various differences.
A proverb, unlike idioms, is almost always a complete sentence, e.g. "A friend in need is a friend indeded". Metaphores are more complicated. It could be a phrase (like idioms)

Couldn't an idiom be a stock metaphor?
Bye, FB

Mrs. Palmer, in her way, was equally angry. 'She was determined to drop his acquaintance immediately, and she was very thankful that she had never been acquainted with him at all'. (Jane Austen)
Students: Are you brave enough to let our tutors analyse your pronunciation?
...what's the difference between idioms, proverbs and metaphors?

It is a metaphor when someone speaks of one thing (or person) as being something else, in order to convey some aspect of its nature, without explicitly stating it. If I say "John is a brick", meaning that he's solid and reliable, that's a metaphor. If the comparison is explicit, as in "John is like a brick" it's not a metaphor but a simile.

An idiom is an expression with a meaning or grammatical structure that can't be understood based on its individual words. It seems like a wrong usage if you think about it, but is widely accepted anyway. It is typically just a few words, not as much as a whole sentence. "Head over heels" meaning upside-down is an idiom.

A proverb is a brief piece of "folk wisdom". Typically one complete sentence, it expresses some form of advice; many people can quote it exactly, but typically it is not known where it came from originally. (The Bible includes a book called Proverbs, and I presume some of them come from there, but I have never read it.) Example: "Marry at haste, repent at leisure."

Mark Brader "Metal urgy. The urge to use metals. Toronto That was humans, all right." (Email Removed) Terry Pratchett: Truckers

My text in this article is in the public domain.
Subject: Difference between idioms, proverbs and metaphors So I ask, what's the difference between idioms, proverbs and metaphors?

With respect, this is like asking the difference between a camel, a screwdriver, and a pot of tea. As other people have explained, they are three quite different things.
Usually.
But they do sometimes overlap. For example, if someone said, "I'm an early bird", that would be an idiom, a metaphor, and a proverb (because the early bird always catches the worm) all at once.
Peasemarch.
Students: We have free audio pronunciation exercises.
A proverb, unlike idioms, is almost always a complete sentence, ... Metaphores are more complicated. It couldbe a phrase (like idioms)

Couldn't an idiom be a stock metaphor?

You mean like "All we like sheep have gone astray?". Until the cows come home?
Seriously though, the second of these is not really a metaphor, but I would say does classify as an idiom.