I currently have two monitors, but I want one more. The width of my computer table is however full, so I thought I would add one above my existing monitors. The backs of the monitors are close to a wall, so I thought of some kind of wall-rig. Alternatively I would also be very happy with some kind of tripod that will sustain the monitor.
What should I buy that accomplishes my goal?
Many monitors have VESA type mounting brackets which allows the use of a standard VESA holder. There are a wide variety of these from wall mount to rack mount to stands which can be attached to a table.
The first thing you need to determine is whether your monitors have a VESA mounting bracket or not. Not all monitors do. It sounds like you are planning to buy an additional monitor so one of the requirements should be that the monitor has a VESA bracket on its back.
The actual mounting gear will depend on whether the monitor has a VESA standard mounting bracket and the weight and size of the display. You will also need to consider how to route the cabling for power and the display signal.
For instance see the results from this Google search: https://www.google.com/search?q=vesa+wall+mount&rlz=1C1GGRV_enUS755US755&oq=vesa+wall+mount&aqs=chrome..69i57j0l5.3149j0j8&sourceid=chrome&ie=UTF-8
