Progress

As you can see in the photo below I have added the supports for the third pole so I will have a strong, triple poled frame. I built these braces a little differently than the first, as you can see from the close-up below.
I cut each brace pole on a 30deg angle, and drilled their mortises at a similar angle around the main poles, so that they meet in the middle snuggly. Only one of them required any real trimming to make snug. Then I make three oval holes in the third pole, set it down over these braces, and secure with a single pin through both braces. This oval hole is on the large side. To keep it from getting too large, and weakening the pole, I notched the peak of the braces.

It all worked very nicely. I don't have pictures of the main frame assembled, yet, but it is almost done. Next will come building the support for the front wheels.