Is it possible to merge the solutions/customization of sandbox1, sandbox2, and sandbox3 to sandbox1?

One of our customer has three sandboxes where as we developed many customizations on each sandbox, now we are trying to do an integration test with all these solutions by placing in one sandbox from other sandboxes, however if we try to do it using bundles will be a time consuming process, so I would like to know is there any way to merge the data/solutions/customizations of SB1, SB2 and SB3 to SB1?, if not what would be the best practice here to handle this kind of use case, thank you,