If you have multiple timers you could add them to a list, then use a random number from zero to the list count to select which timer to start, try something like...

Code:
    Private myTimers As New List(Of Timer)
    Private rand As New Random

    Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
        ' add timers to the list
        myTimers.AddRange({Timer1, Timer2, Timer3})
    End Sub

    Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
        ' get random number between zero and our timers list count
        Dim index As Integer = rand.Next(0, myTimers.Count)
        ' start the timer
        myTimers(index).Start()
    End Sub