You can merge calendars using Google Calendar's import/export function. Please open
http://calendar.google.com on your computer, click on the gear icon in the upper right corner and then on "Settings". Select the "Calendars" tab. There you can find two links, "Import calendar" and "Export calendars":

Please first click on "Export calendars" to download a ZIP file that contains one ICS file for each of your calendars. Then unzip the file and use "Import calendar" to import the ICS files of multiple calendars into an existing calendar. Please keep in mind that the files that you import are added to the calendar, i.e. if you would import the ICS file of calendar "A" into calendar "A" you would have every event twice afterward. But you can e.g. import the ICS files of calendar "B" and "C" into calendar "A" to have the events of calendar "A", "B" and "C" afterward in calendar "A".
Please try this and don't hesitate to contact me again if you have any problems or questions!
Kind regards
Dominique Gunia