On the iMac, go to Messages>Preferences>Accounts>Your Account. Make sure that your Apple ID email is enabled. Then check - You can be reached for messages at - and choose how you want to receive your messages. Check the iPhone number or the email address that you want to use.
I don't have an iPhone but i assume that it works pretty much the same way as it does on the iPad and iPod Touch. Settings>Messages>Send and Receive at>You can be reached by iMessages at .... You have to use the same email address or you can use your iPhone number as your contact number.
Yes you can use all of them, as long as the cell phone number is your iPhone number and it is associated with your Apple ID and as long as your Yahoo email address is your Apple ID email address, or an email address that you added to use in the settings and have selected it for all three devices as the contact email address.
Most people with an iPhone want to use the iPhone number. You might want to consider selecting that in the Send and Receive at" and the "You can be reached by iMessages at" settings.
Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
The Complete Guide to FaceTime + iMessage: Setup, Use, and Troubleshooting
Troubleshooting FaceTime and iMessage activation
iOS: FaceTime is 'Unable to verify email because it is in use'
Using FaceTime and iMessage behind a firewall
iOS: About Messages
Set up iMessage
iOS 6 and OS X Mountain Lion: Link your phone number and Apple ID for use with FaceTime and iMessage
How to Set Up & Use iMessage on iPhone, iPad, & iPod touch with iOS
Troubleshooting iMessage Issues: Some Useful Tips You Should Try
Setting Up Multiple iOS Devices for iMessage and Facetime
FaceTime and iMessage not accepting Apple ID password
FaceTime, Game Center, Messages: Troubleshooting sign in issues
Unable to use FaceTime and iMessage with my apple ID
How to Block Someone on FaceTime
My Facetime Doesn't Ring
Send an iMessage as a Text Message Instead with a Quick Tap & Hold
To send messages to non-Apple devices, check out the TextFree app https://itunes.apple.com/us/app/text-free-textfree-sms-real/id399355755?mt=8
How to Send SMS from iPad
You can check the status of the FaceTime/iMessage servers at this link.
i only have you can be reached for messages on my imac nothing about send and receive at.
Yes. That is correct. I was speaking in terms of what your options are in both apps on all devices. Using the iPhone number on all devices should work, but you can see how it goes.
And by the way ..... when you are done sifting through the twenty-some links in the post above, I would be curious to know if any of them actually helped resolve this for you.