I think they're all beautiful. All three names have a similar feel to me, delicate and refined but also fresh and innocent. I see willow trees and mint.
Ada (ahdah) is difficult to get used to because
Ada (eyda) is a favourite of mine. It reminds me of adders and ardor, but I still think it's pretty and especially in another language where the pronounciation is more natural.
Elma is a bit too close to Elmer/Elmo for comfort but also wonderful and arboreal and it would be such a pleasantly unexpected nickname for
Wilhelmina.
Ottilia is just great, I have been falling in love with
Ottilie pronounced similarly and I'm not sure which I prefer. It's so wonderfully subdued.