Who proposed the concept that the genetic code should be made up of three nucleotides to code for an amino acid?

It was George Gamow, a physicist, who argued that since there are only 4 bases and if they have to code for 20 amino acids, the code should constitute a combination of bases.

 In order to code for all the 20 amino acids, the code should be made up of three nucleotides. 

This was a very bold proposition, because a permutation combination of 43 (4 × 4 × 4) would generate 64 codons; generating many more codons than required.