It is after sunset and after reciting birkat hamazon on the third meal on Shabbos. Is one allowed to drink soda or only water and is water also forbidden?
According to the Shulchan Aruch, after Birkas Hamazon after sunset, we may not eat or drink anything except for water. Although regarding other mitzvos i.e. bedikas chametz, hadlokas neiros, we are allowed to eat a little bit before doing them, havdalah (and kiddush) are different. The reason we don’t eat before doing other mitzvos is not get involved in other things before doing the mitzva, therefore eating a little bit is permitted. However regarding havdalah the reason is so that the first thing we do after Shabbos is to sanctify it, even before eating or drinking and drinking before making havdalah on wine is disrespectful to the havdalah. Drinking water however is permitted, because it is doesn’t have any prominence. According to most poskim this only includes water and not tea, even if it doesn’t have sugar.
According to most poskim plain seltzer is considered water and permitted.
Shulchan Aruch O:CH 299-1, M:B 299-1, Shar Hatzion 299-6, Yeshuos Moshe 1-74(5), Moadim U’zmanim 3-245, Aruch Hashulchan 299-1.
Regarding tea- Shulchan Shlomo 299-1 ftnt. 2, Ohr L’ztion 2-22-7 (permits tea w/o sugar for a sick person), Avnei Yoshpe 4-81(1) in the name of of R’ Eliyashiv zt”l, Kovetz Halachos (Shabbos) 1- 23-6, Halichos Chaim 1 295.
Regarding seltzer- Kovetz halachos ibid ftnt. 7, Halichos Chaim 1- 293, Nishmas Shabbos 298-408.