The northern park's official name is definitely Susannanpuisto. This is a fact as it's the name it has been given by the city of Helsinki and is written in the city plan. Leikkipuisto Sanna is only the name of the playground located within this park. Once again unfortunately that it has the finnish equivalent for park in it to confuse us. Regarding this third name there's now a pending question at the city of Helsinki about how we should relate to that name on that sign you linked. I would wait for this answer before deciding what to do. The parks in the city are their responsibility and they have the final say in what the names are and what signs to put up.