AMS is relatively compact and designed for transfers, as that is the main business model of KLM, the largest tenant. There are no separate terminals, nearly everything is under one main roof.

In both directions, you do not go thru contrôle passeports as you are not entering Schengen- similar to most countries in the world. You will be staying in the non-Schengen part of the terminal and any checked bags will be tagged directly to the final destination in both directions.

Arriving from USA and some other countries, you do not pass thru security controls. From ZA to the US, you will need to go thru security. The whole process is relatively efficient, I see no need to prebook time slots, not sure if this is even possible honestly unless you are originating at AMS. Yes, liquids need to come out......like many other countries. For a non-Schengen to non-Schengen, I would say 1h correspondance time between flights is very safe, so with 3h you will have plenty of time.

If you are flying KLM, there is Lounge 52 available for eligible passengers/tickets.