What do you mean by "Bar?" Are you talking about an Ecstasy tablet?
99.99% of the time, "double-stacked, super ultra stacked, triple-stacked, etc" is just bullshit dealers tell people to make them think whatever they're selling is "twice as strong" or "has three times as much MDMA", etc.
Yeah, its possible that he means a 3mg Xanax XR, but theres no such thing as a "triple stacked Xanax bar". It's just dealer bullshit. Get a look at the pill and Google the imprint to find out what it actually is.
Personally I wouldn't trust anybody who said a pharmy pill was "stacked" in any way, because that just shows you that they have no idea what they're talking about whatsoever.