Final answer:
Ribosomes can read three letters at a time, known as codons, which code for specific amino acids in protein synthesis.
Step-by-step explanation:
The three letters that ribosomes can read at a time are called codons. Codons are sets of three bases, which are read in the ribosome during the process of translating the mRNA into proteins. Each codon codes for a specific amino acid, allowing the ribosome to assemble the correct sequence of amino acids to form a protein.