How to set one State with few value of inputs

i would like to know how can i set one state with few inputs value for the example this is the state

const [userBirthday, setUserBirthday] = useState('')

there are 3 inputs, i want to get e.target.value from each one and conbain them in the state

something like that
**I know the syntax actually not good but it is only to simplify my question

      <input type="text"
           placeholder='Day'
           onChange={(e) => {
              setUserBirthday(userBirthday += e.target.value)
           }}
      />
      <input type="text"
           placeholder='Day'
           onChange={(e) => {
              setUserBirthday(userBirthday += e.target.value)
           }}
      />
      <input type="text"
           placeholder='Day'
           onChange={(e) => {
              setUserBirthday(userBirthday += e.target.value)
           }}
      />   

`

console.log(userBirthday) == Result: 12 September 2020