Not sure if I have understood well your question. My English is far from good.

1 writer will spend 1 time slot in 1 place.
i.e. Cervantes will stay in Bookshop1 at 10 ; and he won't return to that place later

Currently, we have around 350 writers, 100 places, and 15 time slots
If we put all this in single Excel rows (one row for every unique combination of writer + place + time), we now have 1500 rows.

Not all places will have a constant amount of writers every time slot (maybe Bookshop1 has 10 writers at 10, but just 6 at 11).