We can set-up Gmail in different email clients like Outlook and Mozilla Thunderbird. In fact, any email address which lets you enable POP can be set up in such mail clients. So you can use Hotmail and Gmail in Outlook but unfortunately as you cannot import mails using POP, Yahoo cannot be used with Outlook.
To use Gmail in Outlook, first of all, you will need to enable POP in Gmail. To do so, do to settings and click on the Forwarding and POP/IMAP tab and select enable POP for all mail (if you want to import your old mails as well, to Outlook). Next, open Outlook, go to Tools and click on email accounts, select the 'Add New Account' and click next. Then select POP in the type of server. Then you will need to enter the specifications for Gmail. Enter your own username and password and fill out the following details in the respective fields:
Incoming mail server: pop.gmail.com
Outgoing mail server: smtp.gmail.com
Incoming server port number: 995
Outgoing server port number: 465
