From D&D 3.5 to 5.0(5E) - 13 - Saving Throws

Your character will be proficient in two of the three saving throws based on your class which means you will have a saving throw bonus based on proficiency on just those two types of saving throws and no bonus on the third type ever based on your level.  Once again this rule is all about simplification.  We don't need to spend any time when we level up to adjust our base saving throw modifiers.  Instead, when we need to make a saving throw, we roll the d20 and simply add our proficiency bonus if the saving throw happens to be one of the two in which we are proficient.

Page 179.