The seedling is now about 3cm tall and wide.
Should I wait some time before transplanting it to its final pot or can I already do it so it starts expanding its roots wider?
I would be tempted to in a few days. Saying that I forgot the name of the cup thing u growing in but I used to use them and I cut a hole out the bottom and when I saw roots coming out I put it in my final container.
I prefer to start autos in their final pots to minimize stress. I would suggest just cutting out the bottom of your cup and planting the whole thing in your pot.