Hi Jody
Your task numbers 8,9,10 and 15,16,17 are entered as manually scheduled which is fine. If it is correct that the two sets of three tasks can start at the same time then you just need to link task D to the earliest finishing task which you have done in the first section.
What you need to bear in mind is that even when tasks have a duration and resource assigned to them they could still finish earlier or later than the originally scheduled finish dates. To this end you would have to update your dependency's for Task D accordingly. There is no way Project can do this automatically for you.
The other option as explained earlier is to use a Milestone task at the end of each group and link all three tasks to the Milestone. When any of the finish dates for the dependant tasks get changed the Milestone date will change automatically. Then Task D should be linked to the Milestone and that date will also change automatically.
Hope this helps.
Good luck.
Tony(OTPM)