Triangulating a distant ancestor

Hi Lemmy, I wanted to share a story about trying to find a very distant ancestor with DNA.

In the early 1800s, my 3x great grandfather came to Canada from Portugal. I have no record of his birth, his immigration or anything from before 1850, and he definitely changed his name after leaving Portugal. For a long time, I thought that I would never be able to break this brick wall.

But about a year ago, I noticed something interesting in my Ancestry DNA matches. I found several distant relatives with Portuguese ethnicity who consistently share DNA with descendants of my 3x great grandfather. I realized that since I have no other Portuguese ancestors, they must be related through my 3x great grandfather.

In fact, almost all of these matches had ancestors from a small island called Porto Santo in the Madeira archipelago.

I decided to give triangulation a shot since some of the DNA matches had public family trees. Using the Madeira archives website, I built off of their research and extended 3 trees back to the late 1700s. When I did this, I found a few duplicate ancestors, but none that show up in all 3 trees.

Which brings us to now. I've not been successful so far at triangulating my 3x ggf, but I feel so close. Out of curiosity, I made a rough calculation of the likelihood that 3 random people with ancestors from Porto Santo share the same 4x ggf, and it came out to 0.4%. So if I get a triple match in my DNA matches family trees, I can be almost certain that I found a close relative to my 3x ggf.

Any thoughts or advice? This is probably the nerdiest thing I've done to solve a mystery. I'm not sure if it'll work to be honest, but I find it crazy that DNA can make it even remotely possible.