What are the three reasons for man's becoming corrupted in Hamlet's "dram of evil" speech?

I'm not sure Hamlet gives us three clear reasons for evil in this speech. It's one main idea of how men can become evil - and lots of examples of that same idea. I'll paraphrase the speech for you and try to make it clear. Here's the main gist of the speech (I've cut out a chunk in the middle):

So oft it chances in particular men
That, for some vicious mole of nature in them,
As in their birth,- wherein they are not guilty,
Since nature cannot choose his origin,
[...]
Shall in the general censure take corruption
From that particular fault.

Sometimes it happens that, certain men, because of a little tiny part of viciousness in their nature, become vicious overall just because of that one little mole of viciousness. Badness, in other words, can spread out from a little starting point to the "general censure" - over a whole person.

Hamlet gives three main reasons as to what this "vicious mole" in someone's nature might be:

(1)

As in their birth,- wherein they are not guilty,
Since nature cannot choose his origin...

Some men are just born bad...

(2)

By the o'ergrowth of some complexion,
Oft breaking down the pales and forts of reason...

In some men, some natural qualities ("complexions") might grow larger, destroying the capacity for reason...

(3)

Or by some habit that too much o'erleavens
The form of plausive manners....

And some men might have habits which ruin their pleasing manners... and that's how they get corrupted.

Hope this helps!