I was searching for this a particular corner brace/bracket, but it seems that they are impossible to find.
I couldn't them at home depot or at amazon. I'm not sure if they just don't exist or if I'm looking for them incorrectly.
This is it:
I posted a similar question, and it helped because it showed me an alternative, but in this case I want to find this specific bracket that will hold three walls together.
Best Answer
There is this one on Amazon: https://smile.amazon.com/dp/B071W4VQPT You are working with a 20x20 tile and these are 2.25" Roughly
These ones: https://smile.amazon.com/dp/B0195UGD38 They are much larger, though WAY more Expensive.
Though you will see those Brackets. You could use something like this on the Inside with some Galv Bolts or SS Screws to make it look nicer. https://smile.amazon.com/dp/B07CWMSS79
I'd put two on each adjoining edge. so 2 in each corner and 2 on each side on the bottom.