A3 is double the size of A4. It is a popular frame size.
A3 size = 297 mm X 420 mm or 29.7 x 42.0cm or 11.69 x 16.53 inches.
There are no standard poster sizes. Printed material can be printed to any size the client requires. The closest poster size to A3 is 11 x 17 inches. This will make it a bit too narrow and a bit to tall to fit an A3 sized ready frame.
As a professional picture framer I have come across this many times. The customer thinks that a poster sized print will fit into a standard A3 poster frame. They do look at quick glance to be the same size so its always good to check the measurements of your poster before buying a ready frame of A3 size.
The only solution apart from making a bespoke frame up is to cut a narrow surround mount, perhaps of an inch in width. As a fix I always feel this is unsatisfactory. Posters are designed to have a non-fussy framing solution if they indeed are going to be framed at all.