Here are the 3 pieces that need to share a corner.
I sew 2 pieces together first and press the seam - in this case towards the blue piece as the flower piece actually forms a corner of my crazy block.
Then I sew the third (pink) piece to the flower piece, starting at the outside edge sewing towards the blue piece, taking care to stop exactly when I reach the seam joining the first 2 pieces. I sew back for a few stitches then forward again to reinforce the seam. Note: this is NOT how most would sew this. Usually it's recommended to sew from the existing seam at the middle of the 3 point corner outwards in case the sides aren't exactly the same length but whenever I try to do this I wind up fighting with my machine and saying many bad words in the process. So this works for me.
When I open up the seams you can see the junction of the 3 pieces and the remaining seam still to be sewn.
This is the fiddly bit as you may need to tug the material a little to get the 2 sides of the last seam to line up and you will also need to fold the first piece out to make sure you don't catch it in the seam about to be sewn.
Here is the last seam lined up ready to be sewn.
Again I sew from the outside towards the central corner and reinforce the end of the stitching so the corner won't gape or come loose. I don't bother reinforcing the start of the seams as with the addition of more pieces to the growing block all these ends will be incorporated into new seams preventing them from unravelling. However, if you're starting the seam from the centre of the block then obviously reinforce your stitching before sewing towards the outside and no need to reinforce the end of the seam.
Press the seams to one side - in this crazy block the flower piece is a corner, the pink piece forms part of a side and the blue piece is a centre piece so all seams are folded towards it.
The finished product - a 3-point corner.