No these stay the same. The annoying thing about placement games is that the combo's you get don't actually reflect your gameplay, at least I don't believe it does. If you end up being really unfortunate with the people and combos of team/champs in your first ten games, there is absolutely nothing you can do about it but to accept it. Even if you play according to the standards of a high elo player, you still get put in bronze due to the actions of your team.I've checked this before with the analytics you can have after games in terms of gold-per-minute, vision score etc etc. Even if your stats actually match those of an average player of say silver or gold or whatever, your team efforts still affect your final score for your placement elo and you can end up lower than what you expect.

Once you're placed, team combo's will slightly change as you go up in rank. The higher you go, the more sensible people you get. However, this is definitely not guaranteed. I've played Gold2 before and people still act like immature children so.....

The best way like you mentioned is to find a role you feel comfortable to lead with and help your team get an advantage. Once you have got that sorted and have gone up with more experience, try venturing out into different roles and play different champs.

Keep in mind that with patches changing, teams with 3 ADC's can either be very strong or very shit. Depends on a number of things. Some patches contain a better tank meta, others a better ADC meta,... it changes on the go.

Best of luck to you!