Glass cups-May one drink from wine glasses of a non-Kosher bar, hotel lobby, or restaurant:
Drinking non-wine beverages: It is permitted on occasion to drink non-wine beverages from the clean glass cups of a gentile. This applies even if the cups were used in the past to drink Yayin Nesech and were washed in hot water with non-Kosher food. One may even use the glass to drink a hot beverage [so long as the beverage and hot water used does not contain any Kashrus worry]. This leniency applies only towards glass cups, and not to cups made of other materials!
Drinking wine from the cup: It is forbidden to drink wine from a glass vessel of a Gentile that was used to drink or store wine, until it is rinsed well three times.
 See Admur 451:73 for the various opinions regarding glass; M”A 451:48 and Poskim in Darkei Teshuvah Y.D. 121:2 that we are lenient with glass by other Issurim; Admur 451:2 that in a time of need one may use a non-Kosher vessel for cold foods
 See Shach 137:16 on Michaber 137:4 that even if one plans to use it for non-wine beverages, it may not be used “Derech Keva” until it is properly Kashered, as rules Rama 121:5 regarding all non-Kosher vessels, and the proper Kashering for a glass vessel is Shichshuch, as rules Michaber 135:1; See Imrei Eish Y.D. 44; Darkei Teshuvah 137:11
The reason it is only permitted onoccassion: As the one may come to use the cup with wine, which will absorb the taste of the Yayin Nesech and become forbidden. [See Shach 137:15-16]
Other opinions: Some Poskim rule that one may drink non-wine beverages from the cup on an even permanent basis without Kasheirng it beforehand. [Kneses Hagedola 137:23; Chochmas Adam 81:12; See Imrei Eish Y.D. 44; Darkei Teshuvah 137:11]
 Michaber Y.D. 137:4 regarding non-wine beverages that only a mere rinse is required; Shach 137:16 that this applies only for temporary basis use
The reason: a) By other Issurim and Yayin Nesech we follow the opinions [Admur 451:73] who rule that glass does not absorb, and hence is never considered not Kosher. [M”A 451:48; Poskim in Darkei Teshuvah Y.D. 121:2; See article on “The status of glass” for the full details of this subject]; b) One may use a non-Kosher vessel for cold foods in a time of need. [See Shach 137:16; Rama 121:5; Michaber Y.D. 91:2 and Poskim there; Admur 451:2]
 See 1st reason above!
 Michaber Y.D. 135:1 that Shichshuch is required, and 135:11 that Shichshuch requires rinsing of three times, and 137:4 from where it is evident that this applies only when one desires to drink wine from it