For exchanging money at the airport, in Terminal 3, the exchange desk is on the second level of the airport. The arrivals gate is on the first level of the airport. You just take the escalator up the stairs and you will see 2 exchange offices. You can go there to exchange your money.
If you have organized and Airport Pickup through the BestCubaGuide.com website, then it’s probably best to meet the driver first, and then go change your money, so that you don’t miss your driver. The taxi driver will be waiting for you at the arrivals gate (ground floor) when you arrive.