You can't match feed in HTTP_HOST instead you need to use Match URL.
You have to match /feed/testing.xml in match URL which will yield here R:0 Match your patter if it has www3 and take C:2 which will give you URL without www3.
<rule name="fix feeds" enabled="true" stopProcessing="true">
<match url="^feed/.*" />
<conditions>
<add input="{HTTP_HOST}" pattern="(www3\.)(.*)" />
</conditions>
<action type="Redirect" url="http://{C:2}/{R:0}" appendQueryString="false" redirectType="Permanent" />
</rule>