A sequence of three nucleotide called triplet codon is the basic unit of the code relating DNA to protein because we have four nucleotide which can code for 20 amino acids. So if we take a doublet code it can code for only 16 combination (4²=16) which is not sufficient for 20 amino acids.
But when we take a triplet code it can code for 64(4³=64) combination which is sufficient to code 20 amino acids found in human body. So this was the reason for scientist to establish three nucleotide code for a single amino acids. Later it was found that genetic code is universal and in every organism it is same for a particular amino acid.
