1) Never say you have a trick down 100% of the time, you will become lackadaisical.
2) Try to add some grabs to your 3's, simple mute grab to start you off. It is a very different feeling spinning with a grab
3) Backflips are about nutting up, personally i believe they are easier than 3's. You can control your rotation and spot your landing easily.
4) Front flips should probably come after backflips
5) Corks/and Bios feel very different and you should have 5's before you start attempting them. Often I over set a cork 3 and have to swing my feet round and land switch (basically cuz i suck at them) but if i couldn't do 5's then i would be fucked.
This is just my personal opinion should in no way be taken as a guide for a way to progress. Progressing and the method used is subjective and there are an eclectic variety of factors to consider. I just thought I would try to help. Good luck