Mail

I want to start using Mail on my MacBook.
I have 3 email accounts I'd like to set up.
2 are Gmail, they are set up but only one account transferred the existing messages from my Gmail inbox into Mail when it was created. Can I get the other messages?

Also, the third account is my old college email. It's an .edu
Where do i get the information (like outgoing server) to set that up too?

And most importantly,
How do I set it up so that when I read a message through Mail, the next time I actually log into Gmail through the same account, that message will already be marked read?

For the GMail accounts you will simply have to check the settings your using as to why you can't receive mail to the second account. Mail typically attempts to validate your settings for a mail account prior to use. Did you receive any messages from the server? Is Mail giving you any indication as to why it's not working.

For the college account, you have to determine what type of account it is, POP, IMAP etc. If you were using a different client, typically you can transfer your settings to Mail and it should work fine.

To get the behavior you are describing in your last question, you need to set up your Gmail account as IMAP in Mail. You can consult GMail's help on how to do this.