Originally Posted by
corporate-wage-slave
At best you would only have about 30 minutes in the lounge on that sort of timing. You would follow the signs for T3 Flight Connections, where your boading pass won't work at the gates, so you will have to explain you wish to use T3 before going to T5 and if it's a peak period in security I would expect you to be turned away - HAL security may well take a dim impression of this.
Are there gates these days? My (very hazy) T3 memories recall the transfer desks being post security and thus no boarding pass check, as one may not have had the opportunity to obtain one.
That aside, I certainly agree with the conclusion that given the time available this really isn't worth the messing around, and the OP is best served by proceeding to T5 and enjoying Galleries First.