JAVASCRIPT

yup js at least one field required

var schema = yup.object().shape({
  a: yup.string().when(['b', 'c'], {
    is: (b, c) => !b && !c,
    then: yup.string().required()
  }),
  b: yup.string().when(['a', 'c'], {
    is: (a, c) => !a && !c,
    then: yup.string().required()
  }),
  c: yup.string().when(['a', 'b'], {
    is: (a, b) => !a && !b,
    then: yup.string().required()
  })
}, [['a', 'b'], ['a', 'c'], ['b','c']]) // <-- HERE!!!!!!!!