1. 代码方式

验证有效

toolbar.setOverflowIcon(getResources().getDrawable(R.drawable.actionoverflowbutton));

2. 主题方式

参考：https://blog.csdn.net/u013185164/article/details/48157245

<resources> XML

    <!-- Base application theme. -->
    <style name="Apptheme" parent="@android:style/Theme.Holo">
        <item name="android:actionOverflowButtonStyle">@style/OverFlow</item>
    </style>

    <style name="Apptheme.material" parent="Theme.AppCompat.Light.DarkActionBar">
       <item name="actionOverflowButtonStyle">@style/OverFlow</item>
    </style>

    <!-- Styles -->
    <style name="OverFlow" parent="@android:style/Widget.Holo.ActionButton.Overflow">
        <item name="android:src">@drawable/actionoverflowbutton</item>
    </style>

</resources>