Database Offset & Limit without order by?
Your syntax won't work with PostgreSQL, but I can answer your question for PostgreSQL anyway:

Your query will often work as you intend because the three statements will often use the same execution plan. There is no guarantee for that though.

One possibility where even a sequential scan in PostgreSQL will return a different order is if there is already a sequential scan of the table in progress: Then PostgreSQL will start scanning where the other statement is currently scanning the table, i.e. somewhere in the middle, so that both scan can execute somewhat synchronized to reduce the I/O operations necessary.