This reaction is called a condensation reaction. It is also known as a dehydration reaction.
When three fatty acids become attached to a glycerol molecule, a triglyceride is formed. The fatty acids become attached to the glycerol through a condensation reaction named so because 3 water molecules are formed from the 3 OH groups from the fatty acid chains and 3 H atoms from the glycerol.
The bond formed between the fatty acid chain and the glycerol is called an ester linkage.