Settling "How to answer source of funds?" means knowing which question is being asked. Money goes in with almost no questions and comes out with several, and that asymmetry catches most people at their first withdrawal. A casino is obliged to know who it is paying, where that person lives and where the money being staked came from, and it usually asks at the moment it is about to pay rather than when the account opened. Knowing which three checks exist, and what satisfies each, turns a stalled payout into a short errand.
Proof of address is the second, and the document that satisfies it is dated. A council letter, a bank statement or a utility bill from the last three months carries a name and an address on the same page, which is the whole point of it; a bill from last year does not. Screenshots of online banking usually pass where they show the bank's name, the account holder and the address together, and fail where a balance is all they show.
The identity check is the simplest. A government document with a photograph, a full name and a date of birth settles it: a passport, a driving licence or a national identity card. The name on it has to match the name on the account exactly, which is where most rejections come from — a shortened first name, a maiden name, a middle name present in one place and absent in the other. Photographs of a document are read by a person or a machine, so the corners have to be in frame and the text legible rather than washed out by a flash.
The order matters less than the timing. Identity and address are often asked once and never again; source of funds arrives later, usually when the totals passing through an account cross a threshold the operator does not publish. A payout that has sat pending for days while nothing was requested is worth a question to support, because a check waiting on a document nobody asked for is the commonest cause of a stall.
Identity: a photographic document, name matching the account
Salary, pension and a house sale as ordinary answers
Proof of address dated within three months
Pending payouts where nothing was requested
Online banking screenshots, and what they must show