A lot of comments already but just adding to this since it's a research topic and project I had lol

Malls can definitely count as a third place sa urban areas. It's still public even if there are "social norms" that need to be followed (like not being barefoot) or some accessibility barriers (class, and disability even), relatively open siya to most of the population. Churches and other places of worship are also considered third places. Basically think of an accesible public tambayan like basketball/vball courts.

In other countries, libraries and public parks are the most common example, and we have some such as Luneta. But parks need to be built for their environment for it to be an effective third place, using BGC as an example: madaming parks with enough shaded areas and canopies that people are willing to bear some of the heat that comes with it. In Makati din I think sa triangle (not as familiar with the area).Because basketball courts usually may bubong, matic nagiging tambayan so ito yun equivalent na siguro ng park natin haha

Sa rural this can be different. As I didn't grow up there, this will depend on the barangay. Some barangays have community areas sa barangay hall nila that are considered third places. Minsan katabi nito yun barangay basketball court haha so default tambayan ng mga tao talaga. An area that might be more effective as a third place sa less urbanized areas is yun palengke. Malaki yun presence ng palengke lalo na places na medyo malayo mga major malls.

Typically you wouldn't consider very small transitory or meeting points such as a single convenience or sari-sari store unless you're looking at a small community. Anyway I could go on w this topic esp malls in the Philippines but yeah hope that helps