In the rules of what edition of Kingdomino?
Initially 3 players games were played as you described, but then there was a post on Kingdomino players group:
"Dear Kingdomino players,
There was a rule improvement for 3 players games in Queendomino, which was included in Kingdomino latest editions: play with all the dominoes, draw 4 each turn, discard the leftover domino.
This rule is great, so we just implemented it, with Bruno Cathala agreement
All 3 players games are now played this way.
Enjoy the game!"
And 3 players was marked as not recommended before the change happened.
It may unbalance the game in the view of some players, I guess. For me it is totally irrelevant.
If you really dislike this rule, try to persuade admins to make it an option, not a necessity. (I guess it should be done in Suggestions, I think admins rather don't look at forums games.)