Ask.
Really, all you need to do is log into the Windows Phone Dev Center, contact support and submit a ticket with the subject “Phone Unlock”.
Give some reason you need more than 3 devices. Here is a list of good sample reasons:
- I have more than 3 devices.
- I need to test on more than 3 devices.
- I want to wallpaper my house with Windows Phones.
- Each of my 8 cats writes Windows Phone apps and needs a device to continue doing so.
Really, anything approaching a reasonable justification will be accepted and you can get your phone limit increased.