When the ranked reset starts the highest a person can be placed is Platinum. That means that all the Diamond, Master and Challenger players get thrown into Platinum at the beginning of the season. This causes plat players to be thrown into Silver and Gold, Gold players to be thrown into Bronze and Silver, and pretty much all Bronze and Silver players wind up in Bronze. Everyone drops, usually at least a full rank (i.e. Gold 1 to Silver 1 minimum.)
You finished Bronze 3. Even if you went 10-0 you probably would have been placed in Bronze 5 because everyone usually drops a full rank (if there was a rank below bronze such as tin you would have most likely wound up in Tin 3.) Since there's no rank below bronze you unfortunately get stuck in Bronze 5 because there is nowhere else to go.
The other important thing to note is that your first couple games in your placements affect your MMR A TON more than you last couple games in placements. For example, winning your first 8 games and then losing your last 2 will result in you placing in a higher rank than losing your first 2 games and winning your last 8 even though you wind up with the same record over 10 games.
Take a deep breath and chill. If you have the skills you'll be able to climb out. I've climbed out of bronze quite a few times. All you have to do is gain 200 LP and you're back to your season 6 rank. Players at higher ranks who get dropped anywhere from 1 to 3 full ranks have to gain 500 to 1500 LP to get back to their season 6 rank. The ranked reset hurt you a LOT less than them so I'm not sure why you're complaining.