how to go downtown from osaka airport

I will go osaka early of March. How can I go downtown from airport to save money? Can I buy 3 day pass in HK ?

Thanks a lot !

In case you refer to Osaka's international Kansai Airport:
http://www.japan-guide.com/e/e2033.html

In case you refer to the domestic Itami Airport:
http://www.japan-guide.com/e/e2433.html

Can I buy 3 day pass in HK ?

In case you refer to the JR West Pass or Kansai Thru Pass, yes, both passes can be bought in Hong Kong.

from the airport to Downtown you have a choice between various trains and buses
where in Osaka are you going? Kita, Minami, OTHER AREA? have a look at the travel tab on this page (at the top) then on transportation and also look at Osaka info by entering Osaka in the search window to the top left. Some passes are for 1 day, 2 days, days, 4 days. you can buy them at the airport. check passes in the transportation pages.