the situation arrived when a player had one reserved card but already 3 joker tokens and wanted to reserve another card. can he receive a gold token even if he already has 3 in his hand?
Yes, you can have more than 3 joker tokens.
In the list of things you can do on a turn, the rules say:
- Reserve 1 development card and take 1 gold token (joker)
The rules later restrict when you can reserve a card:
Players may not have more than three reserved cards in hand
There is no mention of a limit on the number of jokers*. There is only a limit on the number of reserved cards in hand.
*Note that jokers do still count towards the normal 10-token limit. If you are at 10 tokens already, you can still reserve a card and take a joker, but you will then have to discard a token to get back down to 10.