To start what I'd do is change all texts to Standard. This should set them to the default the new DogTag3 uses.
Ctrl+right click a pitbull frame to get the config up.
For player health: Player-->Texts-->Health. Set style to Standard. If it was on Standard select something else then back to Standard. The custom field will show the actual tags in used but if you want to manually edit them put the style to custom.
Do this for each text field that's showing unknown or undesirable results. This should at least get you in a working state again until you have time to adjust them to your liking.
Sorry Cralor. I guess not all texts have a Standard option. Just set all of them to one of the default ones whether it be absolute, standard, percent etc... That way your frames should at least work properly.
Now for customizing for Target, Guild, Level, Talents etc... did you have all these in Pitbull frames or in your tooltips with CowTip? How was this information shown before?
If ALL your tags are showing up as "Unknown tag <whatever>", even the defaults, that means you are getting a DogTag error and it's not loading properly. Update all your addons, and if you still have problems check your BugSack/Swatter/whatever and tell us what the error message is.