[SalesForce] How to call one function from another function in same javascript file in lightning web component

Are the areas being re-rendered all separate from each other? It sounds to me like you might need to ensure they're not affecting each other and potential element IDs through the use of <apex:actionRegion> elements, something like this:

<apex:actionRegion>
  <apex:actionFunction name="actionFunction1" action="{!action1}" rerender="areaToRerender1"/>
  <apex:outputPanel id="areaToRerender1">
    <!-- output fields or whatever -->
  </apex:outputPanel>
</apex:actionRegion>

<apex:actionRegion>
  <apex:actionFunction name="actionFunction1" action="{!action2}" rerender="areaToRerender2"/>
  <apex:outputPanel id="areaToRerender2">
    <!-- output fields or whatever -->
  </apex:outputPanel>
</apex:actionRegion>

<apex:actionRegion>
  <apex:actionFunction name="actionFunction3" action="{!action1}" rerender="areaToRerender3"/>
  <apex:outputPanel id="areaToRerender3">
    <!-- output fields or whatever -->
  </apex:outputPanel>
</apex:actionRegion>

Alternative Approaches

1. I'm guess you can't do this or you would have done, but you might be able to use a single action function element and just use that to trigger one of more methods in the controller. You can re-render multiple elements with one action function element.
2. Have a look at Javascript remoting. Remoting is typically much faster and easier to work with from a Javascript point of view. The downside is that updating DOM elements is left to you, if it's just a couple of messages or similar then jQuery will probably be the easiest way forward, otherwise check out something a bit more involved, I find KnockoutJS to be good for simple one-page scenarios.

We can access directly.

<apex:page controller="OppsController">
    <apex:form >
        <c:Test></c:Test>
        <input type="button" value="test" onclick="test();"/>
        <script>
        function test(){ 
        function2();
        }
        </script>
    </apex:form>
</apex:page>

Component

<apex:component >
    <script>
   function function2(){
   alert('in component');
   }
 </script>
</apex:component>