The Nintendo 3DS eShop has no native wishlist. Here are practical workarounds:
Doesn't sync to web, but it's the only native "watchlist" mechanic.
The 3DS eShop closed for new purchases March 27, 2023. You can still re-download owned titles and use "Follow" for sale alerts on already-purchased games, but you cannot buy new ones. Wishlists are now purely for reference or tracking physical/used purchases.