How can you identify the amino acids in a tripeptide?

Unfortunately, I can not type in the exact problem that I need help with. I was given three tripeptide structures and asked to identify the parent amino acids. I am supposed to enter the answer in the program using the three-letter abbreviations. For example Ser-Ala-Gly. I can look up the amino acids and identify what the structures look like but when they are joined with others in a structure, I cannot tell what amino acid is what since the tend to lose either a hydrogen or an oxygen when bonded. I am so confused and I have tried looking up similar structures, but I have not had any luck.