We have an agreed judgment on two of the three persons named in the original petition. How do I process a default on the third person that we did substitute service on?

If you have perfected service on the defaulting party (posting, publication, or however you did it), how about just adding a paragraph in the judgment to the effect of "so and so was served by such and such method but did not appear and wholly made default" You'd need to include of certification of their last known address. Or if you dont want to modify the already completed judgment, you could draft a different default judgment specifically for absent party

You could also move to sever the defendants with whom you reached the agreed judgment so that judgment is final, then proceed separately against the remaining defendant.

I have done it like t_walker suggested. Look at the civil rules. They call for the appointment of an attorney ad litem, who is supposed to make sure the process for substitute service was followed. I am pretty sure the TDCAA Guide to Asset Seizure & Forfeiture has the forms to file the motion for default J and appointment of attorney ad litem. If not, I might be able to find some old ones I have used.