Welcome to the Cake Decorators Q&A
Is it food safe to put jam out of a jar on to the layers of your cake if it’s going to be on the side for 3 days?
Is the cake also covered in icing? If it is, yes you can do that as long as you keep the cake in a covered cake box stored in a cool place. If there is no type of covering on the cake eg, buttercream, ganache or icing, cover the cake up with clingfilm so it's not exposed to airborn microbes which could cause mould on the jam. On the whole though, the sugar in the jam will keep it preserved.