I wonder if there is any difference in the meaning among three sentences.
1. Swimming is what I love most.
2. Swimming is the thing I ㅣlove most.
3. Swimming is something I love most.
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.