SnowSkiSunSea

Yes, the taxes and fees are not revenue to RCI they are pass through to the governments involved. I almost think it is a mandatory refund and you don't have to ask (I am sure they submit an official manifest along with $X per person to each government) but I would ask at guest services to make sure they process it.

If they will let RC about only two people in the room after boarding will they still get the tax and ports fee back  (+ gratitude) ?

It is a thing that cabins for triple/quad occupancy are not (usually) offered for double occupancy and that if you are changing your reservation to double occupancy from triple occupancy they MAY make you change cabin and MAY also reprice the cabin to current rates.
Some people (including me) have been told that by RCL when inquiring. In my case I ended up keeping the reservation as a triple since the price for two of us in a double was going to be more than for three of us in a triple, ultimately my wife ended up joining us so no harm no foul.
On a separate occasion I didn't call to ask about it but at the last minute my wife wasn't able to go, so son and I showed up at the pier and checked in with no issue. They refunded her taxes and port fees after the cruise.
Unless you will save significantly by cancelling the third person, just don't say anything until you check in at the pier. At that point they will not be able to move you to a new cabin and you will get the small refund of taxes&fees still.

If the third person isnt going and you aren't interested in a price change, just leave things as they are.  RC does not care if the people booked actually show up or not.

I have seen in the past where the booking engine will not offer up cabins that sleep 3 as available to book if you only select 2 in the cabin.  I don't know it this is still the case.

The number per cabin is usually stated as a maximum not a minimum.  Normal occupancy is viewed as two per stateroom.  Usually the only “penalty” is if you are one.  The third person in a cabin is usually a lower rate.  It should be possible to drop the third person and keep the stateroom.  It sounds suspicious that they will move you to another stateroom if you have at least two.  There must be more to this story.

If you are happy to pay for the third person who will not be joining you, there's no need to even mention it until you actually board the ship.
Just tell them it was a last minute thing, the person was unable to come, and it will be too late for them to move you to another cabin anyway.
The only time I would bother telling them in advance would be if I wanted to get some money back.