I donated 200+ R-Gacha/pudding/peromin/glaases one at a time ZERO SR-Girls so far.
Donating 3*R-Cacha very often give a R-Gacha/pudding/peromin/glaases that can be donated so cost is more like 2*R-Gacha than 3.
Using 3*R-Gacha each time have costed me 200 R-Gacha/pudding/peromin/glaases too but it gave me SR-Girls times.
So it costed me on average 50 R-Gacha to per limmed SR-girl.
Too bad all 4 went to my testing accounts I dont really use.
I made some account to test how to best get the limmed girls.