Spider mites are related to ticks and spiders and have 8 legs
This is a twin spotted spider mite, the most common type:
This is red spider mite:
After a bad infection and they are established they start to make webbing:
You dont have this...
This is a fungus gnat:
Looks like the first and second images you posted. In their larvae stage they can be damaging to the roots of your seedlings. In adult stage they are mostly just a nuisance. They are very easy to control with some diatomacious earth covering the tops of your pots (0.5 - 1cm layer), and feeding water and nutrients from the bottom rather, to let the top dry out.
Aphids can come in many colors and sizes, there are lots of different types of them. I'm not sure if the third pic is an aphid but thats my guess. Just hope that they are not root aphids which are terrible.