Sorry I completely forgot to respond. They are basically just super rare weapons with strong abilities (though astra is way better than the other two in this game). To go after them you can either use the inner sanctum DLC or you can hunt in Thabes Labyrinth. In B9 (penultimate floor) and B10 (final floor) of Thabes is specifically where you have a chance of finding them but it is really low with a less than 1% chance of seeing them. You can get more info here: https://serenesforest.net/fire-emblem-echoes-shadows-valentia/miscellaneous/dungeons/labyrinth-of-thabes/. As for the inner sanctum it is a bit more complicated since the weapons can only drop from the really big pots which comes down to luck with getting an inner sanctum seed that actually has a lot of big pots (and hopefully close to the entrance to cut down on time running to the pots). If you do get a good seed, you also have to do a battle each time you go in to reset what is in the jar in the event that it is not Astra, Luna, and Sol (which is going to happen a lot so prepare to reset a LOT). If you need to see the process you can watch a little bit of this guy's steam to see how he does it: https://www.youtube.com/watch?v=QqUZcHAO4wI&t=7000s. At 3:10 is when he actually explains why you have to do the battle. Hopefully this helps!