Triplets arent all born in one single rush. One baby is born, then the next, then the next. They can be anywhere between a minute to an hour apart. Unless you do a C-section, in which case they are generally like thirty seconds apart. 

SO, Lucan is the eldest cause he was born first, their middle brother is the second oldest, and Nessie is the third, being born last. 

I myself am a Twin, and was born three minutes before my brother, so I'm the oldest.