The first two convey the same meaning and are in common use. The second is slightly classier.
The third is illogical as the use of 'something' implies one of a range of activities, of which swimming is only one, all of which you love most. 'Most' is a superlative and can only apply to one thing in this context.
You could say 'Swimming is something that I love more than anything else.'
You could also say 'Swimming is what / the thing I love the most.' That would be grammatically correct.
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