If you try to specify steps attribute there are some difficulties moving the cursor. Try this code and move the cursor to position 1.
<asp:TextBox runat="server" ID="approximation" />
<asp:TextBox runat="server" ID="approx_boundControl"></asp:TextBox>
<ajaxToolkit:SliderExtender ID="SliderExtender1" runat="server" BehaviorID="approximation"
TargetControlID="approximation" Minimum="1" Maximum="3" Steps="1" BoundControlID="approx_boundControl"
EnableHandleAnimation="true" />
Comments: This ticket was filed for a pre-15.1 version of AJAX Control Toolkit. If this is still an issue in v15.1 or later, please file a new issue.
<asp:TextBox runat="server" ID="approximation" />
<asp:TextBox runat="server" ID="approx_boundControl"></asp:TextBox>
<ajaxToolkit:SliderExtender ID="SliderExtender1" runat="server" BehaviorID="approximation"
TargetControlID="approximation" Minimum="1" Maximum="3" Steps="1" BoundControlID="approx_boundControl"
EnableHandleAnimation="true" />
Comments: This ticket was filed for a pre-15.1 version of AJAX Control Toolkit. If this is still an issue in v15.1 or later, please file a new issue.