The most simple type of machining, where the workpiece is fixed in a single position. Movement of the spindle is available in the X, Y and Z linear directions.
3-axis machines are typically used for machining of 2D and 2.5D geometry. Machining of all 6 sides of a part is possible in 3 axis machining but a new fixturing set-up is required for each side, which could be expensive (more on that below). For a single fixture setup, only one side of the part can be machined.
Many complex and practical shapes can be manufactured by 3 axis CNC milling, especially when in the hands of a world class CNC machining facility. 3-axis machining is best suited to manufacture of planar milled profiles, drillings & threaded holes in-line with an axis. Undercut features are possible with the use of T-slot cutters and Dovetail milling cutters.
However, sometimes the designed feature physically cannot be manufactured by a 3-axis machine, or the feature might be more economically viable to machine with a 4 or 5 axis machine.
Features not possible in 3 axis milling include any features on an angle to the X-Y-Z co-ordinate system, even if the feature itself is planar. There are two types of angled features you can design, and understanding the distinction between them is important when designing parts for CNC milling.