Established Byler but it's both of them realizing at the same time they're both also in love with Dustin, having their "Oh." moments.
Not even two seconds after though both Will and Mike feel immediately guilty for even thinking like that about their friend, especially since they're both in a relationship as well.
So they both sort of let their guilt eat at them for like a week or two straight, both of them staring at their ceilings in bed going over all the scenarios and possibilities that would occur if they told the other about it.
Both assuming the worst but feeling so immensely guilty about the whole thing so much that the next time they have a date or have an agreed upon hang out they try to tell one another about it and after both of the boys sitting in silence for the first five minutes of the date/hang out it goes something like this;
"I have something to tell you.."
They both giggle and after a minute of debate they both agree to say it at the same time, both yelling out-
"You mean..,like- A crush?"
Once they both confirm they other actually does like Dustin they talk about it some and they both agree that they still would like to date but they'd also love to date Dustin as the three of them. After a very deep and very long conversation about it they both end up deciding that to make this plan work they'll flirt with him. Any chance they can get.
See, this plan is fine and all but they both failed to realize that- Dustin is absolutely clueless. He's totally oblivious to the fact either boy has a crush on him and it's not like he doesn't think that he's allowed to like boys- No, Dustin is very open about his sexuality. (Bi/Pan trans guy Dustin you will always have my heart<3)
He just fails to understand ANYONE could like him in a romantic way.
So he ends up brushing off their behavior as some sort of way of being friendly and it gets so bad to the point where they're both so frustrated that Mike just ends up shouting how they're in love with him and then all three stand in silence, in shock.
And then they kiss and all go on a date and Dustin gets all the love he deserves :33