A couple questions re: joining squares using single crochet

I have a few questions about joining squares using single crochet.

1. The corner of my squares have a chain 3 space. Do I join in the chain space or work a single crochet into the chain beginning with the corner (middle) chain?

2. When I am first joining the squares together, is it preferable to first make a slip knot and join with a slip stitch or should I skip the slip knot on my hook and just go straight into the slip stitch?

3. This is similar to my first question - My squares have some chain two spaces in the middle of the squares, when I'm joining that section, should I 1) work a single crochet into each of the chain stitches 2) work a single crochet over the chain 2 space, or 3) work two single crochets into the chain 2 space

Thank you!