I'll bite and side with
mgarbowski
2017 SKC wins USOC
2017 TFC wins MLS Cup
Give a 2017 best Reg season record/conf/goals scored by header/OGs or whatever you want to use, but keep it in 2017.
Let's give it to DCU on OGs
Then
2018 DCU wins the USOC
2018 MTL wins the MLS Cup
Now we have 2 spots for 2018, just use the same qualification criteria as used in 2017.
That way you have 2 yearly qualifiers. To award 3 spots in one year without it already written somewhere feels like you're screwing a deserving team.