you need to call the function, I've added a hotkey to your code so you can call the func with F10
Code:
#include "ImageSearch.au3"
HotKeySet("{F10}","Start")
$X = 0
$Y = 0
Func Start()
$Search = _ImageSearch('mc1.bmp', 0, $X, $Y, 0)
If $Search = 1 Then
MouseMove($X, $Y, 1)
MouseClick("left")
EndIf
$Search = _ImageSearch('mc2.bmp', 0, $X, $Y, 0)
If $Search = 1 Then
MouseMove($X, $Y, 1)
MouseClick("left")
EndIf
$Search = _ImageSearch('mc3.bmp', 0, $X, $Y, 0)
If $Search = 1 Then
MouseMove($X, $Y, 1)
MouseClick("left")
EndIf
EndFunc
while 1
sleep(100)
WEnd