Saying that teams can have multiple ADCs is a bit misleading.

Yes, we see "marksmen" in other lanes. But we need to address what this even means.

There are only three "ADCs" seeing play outside of bot lane. Quinn is played mostly top, Graves is played Jungle/Top, and Corki is played mid. Graves is the only one I have seen commonly in my solo queue games.

Here's the big thing: While these are considered ADCs, they do NOT play the role of an ADC in their respective lanes. Quinn and Graves are close range, bruiser type ranged damage dealers with burst, and Corki is basically a sustained magic damage carry like Azir or Cassiopeia. You wouldn't see a Jinx or Tristana in any lane other than bot lane, because they are what I consider "traditional ADCs", while the Marksman changes created diversity that allows reworked ADCs to fufill different roles in their team. Quinn and Graves are actually poor choices for bot lane now because their kits aren't even geared towards the bot lane ADC playstyle anymore. Corki is still viable, though his package makes him a better choice mid lane and he has always scaled well with levels.

That is why we see Graves/Quinn often building Steraks and Ghostblade. They are more like ranged "Fighters" than ADCs, and they already have an ADC on their team building pure DPS. Corki builds like an ADC to do magic damage. The only time a bot lane ADC should be building defensively is when they are very far ahead or when they have reached the endgame, which nearly never happens in our meta.

So here are some things that are just flatly wrong that some people imply when they talk about "3 ADC comps". 1.) A team with multiple glass cannon, pure DPS ranged Physical damage carries is viable. 2.) Any ADC can play in other lanes besides the three I mentioned. 3.) ADCs are stronger (Rather, they are just more diverse and flexible)