~snip~
This may not be able to totally hinder an addicted gambler from playing. But to a moderate gambler who is attracted to gambling but still has some control left in him, this might really help. This is an effective mechanism to manage each gambler's gambling hobby. But if we talk of addicted gamblers, that would be a totally different scenario. Gambling is almost unstoppable for this kind of people. Even without a single penny, they will always find a way to gamble, either by selling some properties, or by stealing, or pawning, getting a loan, and so on. So this feature is definitely not for them. What is for them is professional intervention.
No, that will not hinder an addicted gambler from playing. I don't think that will prevent the gambler from spending the money. The player will have more than 3 accounts on the same gambling website because if one account already reaches the limit of spending the money, they will use the other account to continue to gamble.
There may be exclusions based on IP address and not just on accounts.
Besides that, I don't think that the gambling website will add that features in their site because their purposes to create the gambling site is to get the money from the gamblers especially from the gambler who cannot control themselves in spending the money.
This is not just all about the gambling websites or casinos, online and physical. This is also about legal gambling regulations. The gambling site may not have any choice about it. Self-exclusion feature is not actually a new idea or concept. It has already been implemented in various countries. OP has already mentioned that in Belgium gambling sites are required by law to have self-imposed limits. Australia has also the same legal policy. Other countries have also exclusion policies but more of exclusions as requested by family members and not by the gambler himself/herself.