Day 3 means that I can show you how to join as you go when you come to working 3 corners together.
On square 3, Granny Shell your way across to the first corner as per the picture below
Now insert your hook into the 3 chain corner space of the Granny Square directly above
WOH and pull through to the front, then make a chain by slipping the wool you have just pulled through, through the loop on the hook
That's 1 chain of the 3 you need for a corner space
Now you have joined Square 3 to Square 2
As there are always 3 chain spaces in a corner, make a chain at this stage (so this is chain 2)
Now put your hook into the 3 chain corner space of the square to your left
WOH & pull through
Then slip the wool through the loop on the hook so you have one loop remaining on your hook and you have made chain 3 for that corner space
Now continue to work your Granny Shells (three trebles) all the way around as per the pattern on Day 1
And there you have it!
And here are all 3 squares together so far