How to define data subtype in VBS?

Question

VBScript

Learning to program with VBS? I'm solving a problem in which you need to write a simple function to determine the largest of 3 numbers entered by the user. I wanted to enter a condition that will be met if the data entered by the user is not a number or cannot be converted to a number. I can't figure out how to do it.
Thank you!
Here is what is now:

Option Explicit

Function Task(a,b,c, result) 'Функция, которая записывает наибольшее число в переменную result
  If a>b And a>c Then
  result = a
  ElseIf b>a And b>c Then
  result = b
  ElseIf c>a And c>b Then
  result = c
  Else
  result = 0
  End If
End Function

Dim x,y,z,result
x = CDbl(InputBox("Введите первое число","Задача"))
y = CDbl(InputBox("Введите второе число","Задача"))
z = CDbl(InputBox("Введите третье число","Задача"))


Call Task(x,y,z,result)

If result<>0 Then 
  MsgBox "Самое большое число: "&result
Else
  MsgBox "Условия не соблюдены",,"Задача"
End If