Protection against infection by omicron with two vaccines is minimal (for that matter, it wasn’t great for delta); protection against severe illness is still believed to be substantial. The booster offers substantial protection from infection.

Israel is not, as far as anyone knows, preparing for a fourth vaccine, though it has been speculated about (and realistically most countries have contingency plans in that direction).

For now, it’s not really known how long protection against infection by a booster holds up. It might be an every six month job, it might require a fourth and no more, three might be sufficient. No-one knows.