Won all 10 placements games (solo) and got placed Silver 3??

what was your rank last season

You get initially matched around Silver 5 when you're placed, so that's pretty normal. You'll climb rapidly if you can hold a high winrate.

It also takes your normals MMR into account the first time.

It starts most at silver 4 and then based on your scores it places you. So you did slightly better than a silver 4