There is a way you can feed three trims to each other by doing a mitre in a funny way.
If you get the two horizontal ones to meet, as with a standard mitre, then cut the third to an arrow-like shape, offer the arrow up to the mitre and cut the mitre to all the arrow to slot in it.
If that makes any sense at all.
I call that one, the
triple-arrow-mitre-dan 
If you get stuck I can grab some trim, cut it and take a pic for you if it comes to it, I have some trim handy somewhere from when I used to graft.