The correct term for three nucleotides in a row which encode protein is a codon.

Here's why:

* Codon: A sequence of three nucleotides that codes for a specific amino acid.

* Triplet: While "triplet" can refer to three nucleotides, it's a more general term and doesn't specifically indicate their function in protein synthesis.

Therefore, codon is the most accurate and specific term for three nucleotides in a row that encode protein.