New to pump and forgot to ask a few questions...

Hi guys! I started my T:Slim X2 on Tuesday (my very first pump) and so far I'm loving it! I have a few questions though.

1. I know you're supposed to change the infusion site every three days, but does that include the day I start it? For instance I started on Tuesday, so would I need to change it today (Thursday) or tomorrow?

2. If my cartridge runs out of insulin before my site changes, do I need to use a new cartridge or do I just put more insulin in the current one?

3. I currently have a pretty low TDD (around 20-25) and I have heard of people drawing out leftover insulin from the cartridge and reusing it. Anyone tried this successfully?

Any other tips and tricks would be much appreciated!