Hey🙂
The widest frames on the R3a are 40mm; that's much wider than 28mm. So unless you want to have absolutley no idea what is going to be in shot, then yes you would need the aux finder.
However, as has already been stated, the outside edge of the R2/R2a viewfinder gives a rough approximation to a 28mm filed of view, although of course you wouldn't get any parallex correction. If it were me and I shot mainly wide angle, I would go for the R2/R2a over the R3a.
ManGo