Grammarians say that not only and both should be followed by elements having the same grammatical structure. Therefore all of your sentences are correct. Another option is He is interested in both gardening and collecting stamps.
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It's my understanding that the differences between 'not only A but (also)B' and 'both A and B'
First, in case of 'not only A (,)but also B, you can put comma in front of B.
Second, 'Not only A but also B' has the hidden meaning the writer has his or her heart set on: intentional emphasis on B, compared with A.
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