This script will merge multiple calendars into a separate, isolated calendar.

google_calendar_merge.js

// Google Calendar Merge Script
// This script will merge multiple calendars into a single calendar. Slightly updated from an
// earlier version: http://techingthetech.blogspot.co.uk/2015/04/merging-google-calendars.html
//
// Usage:
//  1. Grab the calendar name and IDs for your target calendar and the ones you want to merge
//  2. Go to Google Drive and create a new Google Apps Script (it will be under New > More).
//  3. Search for the "App Script" add it and allow it to work.
//  4. Create a new script and copy and past this code in there.
//  5. Add your IDs below for the target calender to import to and to import from.
//  6. From the Google Apps Script page go to Resources > Current Project's Triggers. Click the link
//      to add a new trigger. Select "sync", "Time-driven", then the time period you want it to run.
//
// Hints/Notes:
// * You can use the play button the Google Apps Script page to run the app.
// * API documentation is here: https://developers.google.com/apps-script/reference/calendar/
// * Multiday events don't seem to be supported by the API. This script uses normal scheduled events
//      to create multiday events, but this can lead to timezone issues. This script ignores those
//      issues.

function sync() {

  const importCalendarIds = [
    // 'name': 'calendar_id',
    ['SuperSoho', "brh9v6sln9hhhhhhhiookgkgrp8@import.calendar.google.com"], //imported airbnb calendar
    ['CenterofSoho', "pq1u34v5rhhhhhvgsa3n2p@import.calendar.google.com"], //imported airbnb calendar
    ['Internal', "cal@webso.co"] //internal user calendar
  ];

  const targetCalendarId = 'hostmaker.co_4tk5ffffff3bs8@group.calendar.google.com';

  for (calendarIndex in importCalendarIds) {
    var currentCalendar = CalendarApp.getCalendarById(importCalendarIds[calendarIndex][1]);
    var today = new Date();
    var enddate = new Date();
    enddate.setDate(today.getDate() + 90); // only merges out N days, and only future events
    var eventsToImport = currentCalendar.getEvents(today, enddate);
    var targetCalendar = CalendarApp.getCalendarById(targetCalendarId);
    var existingTargetEvents = targetCalendar.getEvents(today, enddate);
    var stat = 1;
    for (eventIndex in eventsToImport) {
      stat = 1;
      var currentEvent = eventsToImport[eventIndex];
      var currentEventTitle = importCalendarIds[calendarIndex][0] + ': ' + currentEvent.getTitle();

      // skip events that already exist in target calendar
      for (existingIndex in existingTargetEvents) {
        Logger.log(existingTargetEvents[existingIndex].getTitle());
        Logger.log(currentEVentTitle);
        if (
            (existingTargetEvents[existingIndex].getStartTime().getTime() == currentEvent.getStartTime().getTime()) &&
            (existingTargetEvents[existingIndex].getEndTime().getTime() == currentEvent.getEndTime().getTime()) &&
            existingTargetEvents[existingIndex].getTitle() == currentEventTitle
          )
        {
          stat = 0;
          break;
        }
      }
      if (stat == 0) continue;
      if (currentEvent.isAllDayEvent() && currentEvent.getStartTime().getDate() == currentEvent.getEndTime().getDate()) {
        var startTime = currentEvent.getStartTime();
        startTime.setDate(startTime.getDate() + 2);
        targetCalendar.createAllDayEvent(currentEventTitle, startTime, {
          location: currentEvent.getLocation(),
          description: currentEvent.getDescription()
        });
      } else {
        targetCalendar.createEvent(currentEventTitle, currentEvent.getStartTime(), currentEvent.getEndTime(), {
          location: currentEvent.getLocation(),
          description: currentEvent.getDescription()
        });
      }
    }
  }

  var calendars = new Array();
  for (calendarIndex in importCalendarIds) {
    calendars[calendars.length] = CalendarApp.getCalendarById(importCalendarIds[calendarIndex][1]).getEvents(today, enddate);
  }

  // Mario Note:  The following seems to delete events we want created.  Maybe issues w/ multi-day events?
  // No more time to even look at this. Seems super redundant.  Also `sleep(1000)` - takes ages.
  // Not useful in our use case.
  // checks each event in events against eventIndex to see if they are the same
  // if eventIndex does not exist in events delete it
  // will have to have existingTargetEvents rotate through the array of importCalendarIds to check all calendars
  //
  /*
  for (eventIndex in existingTargetEvents) {
    var count = 0;
    Utilities.sleep(1000);
    stat = 1;
    for (eventList in calendars) {
      for (ev in calendars[eventList]) {
        count++;
        if (
          (existingTargetEvents[eventIndex].getStartTime().getTime() == calendars[eventList][ev].getStartTime().getTime()) &&
          (existingTargetEvents[eventIndex].getEndTime().getTime() == calendars[eventList][ev].getEndTime().getTime())  &&
          (existingTargetEvents[eventIndex].getTitle() == calendars[eventList][ev].getTitle()))

        {
          stat = 0;
        }
      }
    }
    if (stat == 1) {
      existingTargetEvents[eventIndex].deleteEvent();
    }
  }
  */
}​

hey! will it merge them in a continuous manner? or just once?

I regret that I don't remember. It's been ages. This was used as a temporary hack at a small, now defunct company. Good luck. Wish I could be more helpful. :(