my apple id is not recognizable on 3 devices, although i can sign in online on icloud

i have three apple device (two iphones and one ipad), under the same apple id.

I can sign in on icloud under the same id.

when i enter the account password on the devices, i get: "your apple ID or password is incorrect".

I contacted the customer care and they needed the proof of purchase which I didn't keep.

Although the three devices have the same apple id, and the account is verified by my mobile phone number too.

I don't know what else to do so that i can restart using my devices, as I'm worried if I get a new one the same problem would reoccur.