try
<asp:Button Text="text" ID="btnTest" OnClientClick="disableAllButton();" runat="server" OnClick="btnTest_Click" />
function disableAllButton() {
var buttonsToDisable = [];
buttonsToDisable.push(<%=Button1.ClientID%>);
buttonsToDisable.push(<%=Button2.ClientID%>);
buttonsToDisable.push(<%=Button3.ClientID%>);
buttonsToDisable.push(<%=btnTest.ClientID%>);
for (var i = 0; i < buttonsToDisable.length; i++) {
buttonsToDisable[i].disabled = true;
}
}