I'm not certain about the lounge access, however I do know that the three main terminals are interconnected via walkways (can take up some amount of time, maybe 20 or so minutes, depending on where you are walking to/from) and the skytrain. Getting to/from the lounge to your gate should be fairly easy, if perhaps slightly time consuming due to the size of the airport

http://www.changiairport.com/imapswf/imap.html

http://www.changiairport.com/c/docum...&groupId=10136