[Hiikomori]

my choices as follows...

Gills: The show has 3 main riders (i know there is 4) and he is the 3rd one to appear.

Zolda: The 4th to appear but he is the third rider of the long-surviving quartet. Raia could be considered the third rider in the early part of the show's running...but you know.

Delta: He is the third main rider gear belt.

Chalice: I admit, I was on the 'Chalice is the secondary rider' camp and I am okay with Garren being the official secondary...despite how insignificant he was in the show after his arc was over.

Todoroki: I haven't watched Hibiki yet but considering the info I have read on the show, he is the third since he works with Hibiki and Ibuki as a trio.

Kabuto: This is tricky. In terms of screentime, Kageyama/Zabee/PunchHopper would be considered 3rd rider but Sasword felt like the 3rd rider due to the timing of his appearance and him being a heroic rider. However, I also feel Yaguruma/KickHopper can also qualify as he feels like a rival to Tendou.

Kiva: Taiga/Saga being the third rider to appear in the present. Otoya is the primary rider in his time period~

Decade: Well...Onodera can be considered the third rider...for the sake of finding one in this series lol.

OOO: The Birth Prototype is the third rider to appear...but I cant decide between Goto or Date being tertiary. Both of them are equal awesome riders as Birth.

Wizard: I guess the first Mage...since she is the third heroic rider in-series.

Gaim: Micchy is the third rider of this series as Ryugen/ZanShin, enough said. The stuff this kid did...sheesh.

Drive's is Chaser. Ghost's is Necrom. Because there like only 3 main riders.
Ex-Aid's is Snipe. Build's is Grease. Zi-O's is Woz. They are the third riders of their respective main quartet. Also that drink ad they always had!

(I seriously wonder if they made 3 riders on purpose since Ghost...for that drink ad...I can't recall if that ad ran during Drive..)

now comes Zero-One. Valkyrie may be 3rd to appear...but I feel the 3rd rider position could belong to Jin. I won't be surprise if what happened in Drive happened in this series since there is some similaries in terms of plot.