I have three bonsai seedlings that are just about 3 months old. They are Norway spruces and appear to be doing well but I was wondering if maybe I need to add more soil (particularly for the third one, see second screenshot). After watering them for some time it looks like the soil level as dropped and I’m worried that if it’s too low the roots/stems might not build a solid enough foundation in the soil.
It looks as if the third one has had some soil washed aside around the base of the seedling so that some fine roots are visible. You can certainly top that up to cover any roots, but not so much it causes a raised level around the stem. When you water, do it with a gentle stream, preferably trickled slowly all over the soil so this doesn't happen again.