Please don't do names with only one letter changed, like Darlene, Charlene and Marlene since triplets may be people who were born at the same time, but they are not copies of each other, even if their bodies are clones.

You can try a theme, like:

Earth- Skye, Waverly and Sandy

Space- Soleil, Stella and Luna

Less Obvious Space- Eliana, Aurora and Cynthia

Organic Jewels- Amber, Coral and Pearl

Crystalline Jewels- Ruby, Opal and Jade

Flowers- Rose, Violet, Lily

Ruling Queens from England- Mary, Elizabeth and Anne

This is just a few suggestions. You can find more if you trawl through baby name sites. I like using this site for when I need a name for a character. 20,000+ Names from Around the World--baby names, pet, pets, sim, story character, meaning, origin, etymology, etymologies, meanings, origins