A blue post from Blizzard on solving Error 3007:
Have you tried joining General chat? That seems to resolve the 3007 disconnections for many. The error is a generic 'connection lost' error, so there could be any number of causes. For people disconnecting within the first 5-10 minutes and very consistently, joining General chat seems to be a successful workaround.
For those people it looks to be an issue with how the game sends data, and how some specific routers work with it (or in this case, don't). It's likely a router support issue we can attempt to mitigate from our end, but it'd be through a patch at some undetermined point in the future. What we need to do now, and ask for your assistance in, is troubleshooting the issue from the user end to try to find a workaround in the mean time.
If joining General chat does not resolve it, again, it's going to be a process for our tech support team to work with you and try to find the cause for your issue. Battle.net/support
If the above fix does not work, try the following fixes.
- Change your TCP and UDP timeouts on your router to a higher number. (If applicable.)
- Also power cycle your modem. (Check google on how to do this.)
- Lastly, try opening up port 1119 in your firewall.
If you have some of your own fixes for Error 3007, or any of these don't work, please post a comment below.