The 3 OADs will conclude the Days anime series, not the series as a whole. Or in other words, there is no planned continuation for the anime after the OADs.
The anime series adapts the manga until the semifinal match of National Championship - Tokyo Preliminaries Block A. The last episode ends just before the start of the final match:
The credits roll as both Touin and Seiseki take the field. There's a lot of high energy and everyone we've come to love throughout the series is watching the game. But we're not! Because the season has ended and we're left with a massive cliffhanger and one last 'Seiseki Fight!'
DAYS Episode 24 (Final) Review: I Want to Play More Soccer With This Team
The planned 3 OADs will draw upon the final match of National Championship - Tokyo Preliminaries Block A, ending the arc and serving as the anime's finale.
Or just as Wikipedia stated,
the television anime's story will end with three original anime DVDs (OADs) in 2018.