A solution to that problem would be to give the main character a map with the location of the three treasure chests. [...]
This or a (obviously not visible) delimited zone for search. If the player reach the limit of the zone, then we hear him think something like, "no, he can't have hidden them this far". Or three smaller delimited zones, one for each chest this time. If the player enter the zone, then we hear him think, "hey, seem to be a good place for to hide a chest, I should search here". Even a map with just some indications can help, two trees with a rock between them is easier to found than something which, seen from far away, can look like a rock or, worse, can't been seen because there's a tree right behind and the colors seem the same.
This way, the player still have to search for them, but it will not take more than some minutes.
For me it's a must have... Naturally I'm nearsighted and astigmatic, and because of my age I start being also farsighted (yeah, it suck to be me). Looking at a computer screen start to be difficult when it come to differentiate few pixels from others ; and the more they look the same, the more it's difficult. So when I have to do quests like this, I must be near to what I search (let say that it must be at least 1 or 2 cm in the screen if the colors seem the same) for to effectively see it. And here, I searched for around twenty minutes and never been able to found the last one.
Sorry, but as good looking and interesting a game can be (and this one is both), I'll not take this must time just for to achieve the first quest. Especially since it's in fact the second ; remember the do
something for me, look around if there's still something useful. I'm not mad that there was nothing to found, but I've already past some time searching in the village, trying the doors, looking if a window can be opened, jumping on the barrels to see if, by any chance, there was a way from the roof, trying to get every object I saw, even the fishes... So, now searching something that I know is here but I'm not able to find, it's frustrating and that's not what I expect from a game.