I played against silvers etc. in my promos, won 8/10 games and got placed in Bronze 3 explain??

Welcome to soft reset.

And challenger player gets plat 1.

it's bullshit but happens to everyone

You actually got a prrety good deal, considering many gold 5 players almost got bronzed. But since you want an explanation:

You don't start placements as your CURRENT rank. Instead they use this formula: (previous season mmr + 1000)/2. That number indicates the elo and division that you will start when you play placements, which will ALWAYS be lower than your current rank. From there, you can only drop. Going 10/10 ensures that rank from the formula. Any lost game will drop you further. Riot's explanation for this reset is that if you are truly good, you will climb back to your original rank because you truly belong there. I don't agree with the really not-soft reset, but it is how it is.

As for dropping tiers, all master and challengers, no matter if they go 10/10 in placements, end up in plat 1/3. There, they still face their usual pile of high elo players, who will also climb from there. It takes a while before everyone is situated at their original (or if they're bad, lower) rank.

There is a fairly good thread that explains this here: http://boards.na.leagueoflegends.com/en/c/gameplay-balance/E9nkYATc-psa-regarding-ranked-placement-and-seedings-for-all-who-are-confused

Lol this your first reset bud? Get used to it.

Thanks guys I guess its starting to make more sense now, and i can climb out of bronze easily so no worries