Top 3 picks are incredibly valuable. I would only really consider moving down in a couple of scenarios. Firstly I have 4 or more rookies I value approximately equally or secondly, I’m contending and the pick can be used to acquire top tier talent to push me over the edge.
Otherwise I will just pick my highest ranked player at 3 and build hoping they are a big part of the future.
Edit in terms of trading confidently, all you can really do is as much research as you can. Everybody could break out, everybody could bust and everybody could be meh.
Another reason I might trade back on reflection is positional strength. Especially if I’m a while off contending, in that circumstance if rb is projected at my spot I will look towards the wr options. If I can build strength there it can buy me time in my rebuild where I don’t feel I’m burning the best years of their career whilst I’m struggling. I really believe in zero rb approaches to kick starting a dynasty rebuild.