$17 billion added to Berkshire Hathaway's Alphabet stake is the number that now places Google's parent company in the third position in Berkshire's equity portfolio. The purchase built on an existing holding. Only two equity names in that book rank above Alphabet.
The $17 billion is the incremental addition, not the combined size of the position. Alphabet, Google's parent, was already in Berkshire's portfolio before this purchase; the add-on alone was enough to lift it to third. That rank means Berkshire now holds more in Alphabet than in every other equity name it carries except two.
Third place in Berkshire's equity portfolio is a named position with weight. The $17 billion figure moved Alphabet there.