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MYSQL fulltext - What's the right way to get the relevance from one column?

This example shows that only one fulltext index is needed. http://www.vionblog.com/mysql-full-text-search-with-multiple-words/

Here are queries from that example:

ALTER TABLE products ADD FULLTEXT(title, sdescription, ldescription)

SELECT *,
MATCH(`title`) AGAINST ('+iphone +case +4s' IN BOOLEAN MODE) * 10 as rel1,
MATCH(`sdescription`) AGAINST ('+iphone +case +4s' IN BOOLEAN MODE) * 3 as rel2,
MATCH(`ldescription`) AGAINST ('+iphone +case +4s' IN BOOLEAN MODE) as rel3,
FROM products
WHERE MATCH (title, sdescription, ldescription) 
  AGAINST ('+iphone +case +4s' IN BOOLEAN MODE)
ORDER BY (rel1)+(rel2)+(rel3) DESC;

And this answer suggests that three fulltext indexes are needed to do the same. http://stackoverflow.com/questions/547542/how-can-i-manipulate-mysql-fulltext-search-relevance-to-make-one-field-more-val/600915#600915

I want to make one column more valuable than others like it's been done in the above query. I'm confused how many indexes are needed for that, one or three? I asked this question on stackoverflow but not answer yet.