Lidl does two types of pannier: a conventional pair, and a three-bag affair where the top bag resembles a suitcase.
A3 fits the suitcase. It also fits the conventional pair, but with little or no margin to fold the top over.
An A3 folder is bigger than A3, of course, so take it with you into a shop.
I don't know when the next Lidl cycle sale is on, but you might find they have something leftover from the recent sale.
(If your folder is sturdy and waterproof, you might be better off just strapping it to the rack.)