Yes, we can combine them all. It is easy. I will help you with that. Connect the backup drive to your older Mac. Open the backup drive, and create a folder on it and name it "To Combine". Then open that folder and create three folders - From Mac 1, From Mac 2 and From Mac 3
Now on the old Mac, go to go to your Pictures Folder (in Finder, click on Go menu and select Home, then open Pictures folder) and find the iPhoto/Photos Library there. Right (or hold down Control) Click on the icon and select 'Show Package Contents'. It will open the library contents in Finder. Now in there, you will find Masters or Originals folder.
Select that Masters or Originals folder, then open the From Mac 1 folder in external backup drive and copy that Originals folder there.
Then do the same procedure to copy the Masters or Originals folder from other Mac to the respective folders in the external drive. Once you done this, connect the backup drive to your primary Mac. Then hold down Option key and open Photos app. Select Create New Library. This will create a blank or empty library. Then click on File > Import from Folder. Then selcet the resective folders from external hard drive. This will give you a combined library. This is the best way, but going to take a few hours to sort this out if you have so many photos.
Let me know how it goes please. If you face any difficulty, please let me know and I will walk you through it