Nienke, Lilly, and Doris, Mara or Eline and Donna.

I think Lilly is really beautiful.
I also find these beautiful and they fit well with it:
- Olivia
- Anna
- Ellie
- Suza
- Charlie
- Rosalie
- Mila
- Julia
- Flore

Good luck with your choice!

I think Nienke and Eline go well together. Especially if you have another name with the -ie sound.
I don't really like Doris and Donna, to be honest.
But I do like Lilly, though.

I really like Lilly and Doris. Nienke doesn't fit as well. Mara fits better.
Mara, Lilly, and Doris.

Nice names.
I think Mara, Eline and Donna are the best combo.

I love it when names have the same number of letters and either all end the same way or all differently, as well as having the same number of syllables. Here are some combos for a set of triplet girls that I find nice:

Aster, Tirza and Lorin
Bodil, Marin and Lotis
Clea, Niké and Yfke
Daphne, Nienke and Benthe
Doris, Pippa and Estée
Eline, Julia and Naomi
Hannah, Lauren and Carice
Isabel, Juliët and Feline
Jade, Loïs and Mila
Jente, Flore and Silke
Lea, Mia and Noa
Lune, Febe and Mare
Mara, Luna and Rosa
Nila, Jula and Ella
Sarah, Chloë and Milou
Sofie, Lotte and Merel
Tessa, Donna and Pippa
Vieve, Minte and Laure
Ziva, Elin and Jade

I think Doris and Mara are the cutest names. Nienke feels a bit outdated to me, kind of cliché. Eline is too plain for my taste. Is Feline still an option?
I think Lilly and Donna are okay and they really suit each other well.
So maybe some new combinations here:

* Lilly, Donna and Estée/Faye/Mia/Rosie//Mae/Ella/Romée/Olivia/Rose/Novée
* Doris and Cato/Fien/Pippa/Juna/Lot/Tessel/Mette/Luuke/Vesper
* Mara, Feline and Linde/Romée/Jinthe/Suze/Veere/Jasmijn/Vieve/Noore

Is there anything in there?