Hey Kazuki,
For calling entity 3 on entity 1 page, you have to call entity 2 first, and based on, what you have chosen in entity 2, entity 3 will have different attributes each time,
For this implimentation,
You can refer to this question, here I have explained the whole scenario, and how we can achieve it,
https://forum.mendix.com/link/questions/119063
Just put all this in the dataview of Entity 1 and then using a microflow on the Save button, you can link entity 3 to entity 1,
Hope it helps!