** HK shares of HSBC 0005.HK rise 2.3% to HK$87.75, highest since April 2

** Stock set for biggest one-day pct gain since April 23; top pct gainer on Hang Seng Finance Index .HSNF, which is up 0.3%

** London-based HSBC Holdings HSBA.L launches $3 bln share buyback after Q1 profit drops 25% due to one-time charges related to business disposals in Canada, Argentina

** YTD, HK stock gains 15.2%

(Reporting by Donny Kwok)

((donny.kwok@thomsonreuters.com))