Is there any halachic basis for this? I’ve seen this done in Chabad houses where people specifically make it a point to learn Torah instead of eating at the time of shalosh seuudos?
I’ve tried inquiring on Chabad websites but they only have an explanation for why one may eat mezonot and not wash.
No one has yet offered an explanation as to how one gets away with purposely not eating anything at all.
The basis for doing this is from the Zohar, that R’ Shimon Bar Yochai learnt instead of eating Shalos Seudos, on erev Pesach that fell on Shabbos. See Magen Avrohom O:CH 444-2, and 290. However most poskim do not agree with this, and that Shalosh Seudos should be performed with bread, if not then with some mezonos or at least fruits. See Aruch Hashulchan 444-6, Kaf Hachayim O:CH 444-18, Mateh Efrayim 597-4. Also see Mayan Chaim 5-8 who talks about this topic at length.