1: Dragon doesn't handle 3 way transitions.
2: There are no tiles in UO for 3 way transitions.
These 2 unfortunate circumstances we can do little about.
I suppose someone could make tiles for 3 way transitions and then some genius like Sydius

could write us a program that can handle 3 way transitions but it sounds like an awful lot of work.
Anywho, bearing that in mind do your best to avoid 3 way transitions as much as possible. I know it is not completely avoidable but it can be minimized. Try to have every terrain area surrounded by something else on all sides. Try to keep at least a border of 2 grass tiles around all coastlines. Then give your map a walkthrough and find the areas that need to be fixed. Use worldforge to place something that looks more natural and then walk the map again.