iMessages sending to the wrong person

mik296 wrote:

We did share an Apple ID but I created a personal one

The fact that you did share an Apple ID is the underlying cause of the problem.

Once the new Apple ID is created, the person who starts using that new Apple ID needed to thoroughly and completely cleanse all her/his iOS devices as follows:

What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

Apparently that did not happen. Do it now if possible.

mik296 wrote:

Why would this be happening if all 3 of us are using the same Apple ID?

When doing something you never should have been doing in the Apple world, I strongly suggest not going through a "why ..." analysis. You're doing something that you should change.

mik296 wrote:

This only started happening about a week ago

The answer is similar to the above. At times, Apple does not strictly enforce things until later. Activation Lock, for example, was not part of the iOS world until iOS evolved a bit. Enforcement of one Apple ID per person is stronger since iOS 12.

I have the same issue, 5 iPhones, 5 independent Apple IDs. We have been using family sharing for year or two, prior to family sharing we all shared an Apple ID.

Since we updated 1 iphone 6 to ios12, the iMessages from that phone send to the wrong person. If I erase and reset all the phones, if I then restore from a backup will the problems also be restored?

Do not restore a backup to an iOS device with a changed Apple ID. That would re-introduce the problem. Read this whole thread, especially the part about cleaning the iOS devices.