It is posible to get the limmed SR-girl donating just 1x R-Gacha each time but I suspect chance is less than 1 in 1000.
So expect to spend more than 1000x R-Gacha if you go this way to get SR-girl.
For me I newer got SR-girl from just 1x R-Gacha in more than 500 tries so far and I only know 1 or 2 players who claims to have got it.

I got limmed SR-girl 5 times from 3x R-Gacha chance seem to be around 1 in 50 tries or 1 per 150 R-Gacha.
So expect to spend 150x R-Gacha if you go this way to get SR-girl.

Other than the SR-girl 3 time 1x R-Gacha give much better rewards than 1 time 3x R-Gacha.

So if you want the SR-girl donate 3x R-gacha but expect if you unluck to spend 300x R-Gacha before you get her.
When you got SR-girl donate 1x R-gacha or wait untill they update with new girls.

N- and R- girl can be obtained from both donating 1x R-Gacha and 3x R-Gacha.

So I would go for SR-girl first by donating 3x R-Gacha until I got the SR-girl.
When I got the SR-girl I would donate 1x R-Gacha if I am missing the R- or N-girl.
Save the rest of your R-Gacha for when they update the limmed girls.
Expect that each set of limmed girls cost 200-300 R-Gachas.
If I got more than 500 R-Gacha you could donate 1x R-Gacha just to get SR-Wild3.

At Japan server first set of limmed girls stayed for 4 months. After that limmed girls changed every month. Collecting 2-300x R-Gacha per month can be hard for free players.