The last player typically get stuck in either the support or tank role. While it's not your job to be forced to pick anything (ELO does not determine what you HAVE to play) it's probably best to try and pick something that will help the teams comp.
When 3 players insta lock carries I just grind my teeth and accept the fact that this will be a high stress game. The best games I have are when the team is extremely talkative about picks from the very start and try to synergize with each other.
let the idiots have their day. Just try not to get to mad at them if they proceed to cost you games and subsequently blame you. Insta lock players really don't care about anything other than themselves and are incredibly fast to pass the blame to teamates who are doing their best to win.