It stands to reason that that they will not be a strong as a square stringer with dementions the same as one surface of the tri stringer. There will be about 60% ( a guess as I didn't calculate it but guessed) of the cross section area compaired to a square stick. What it may do though is let you use a harder and stronger wood with no increase in weight.

From the photo, they are not a 90-45-45 shape, but closer to a 60-60-60. That will complicate things a bit. However cutting them from a sheet would be quite easy, setup you blade at 60 degrees and make a length wise cut, flip the sheet over and make you next, flip and cut until you are done. It would take two cuts on square stock with scrap on both cuts.

Don