Choose Create, Arc, Arc 3 points.
Arc Three Points ribbon bar

Use this ribbon bar to create three point arcs. Click to set each of the three edge points in the graphics window.
This function allows you to enter values in the X, Y, and Z fields in AutoCursor to position any or all of the three edge points of the arc.
You can also create arcs tangent to curves and points. See Guidelines for creating arcs tangent to entities for more information.
Notes:
 Mastercam calculates arcs in a counterclockwise direction.
 No more than two of the points you select can be collinear.
Shortcut Keys
1 – Edit point 1 (edits first point)
2 – Edit point 2 (edits second point)
3 – Edit point 3 (edits third point)
T – Tangent (creates an arc tangent to one, two, or three entities)
O – OK (fixes live entity and exits function) 
Guidelines for creating tangent arcs
The following are guidelines for creating arcs tangent to entities:
 When you enter a point for an arc to be tangent with and that point does not lie on the selected line or arc, Mastercam projects the point along the normal vector of the selected entity.
 Arcs may be created on an extended portion of the selected entity.
 When creating a circle tangent to two curves, the radius of the circle must be greater than or equal to half the distance between the closest points on the two curves.
 Use the Create arc three points function to create a tangent arc between parallel lines.
 When creating an arc tangent to a line or arc with a defined edge point, radius of the arc must be greater than or equal to half the distance between the selected entity and point at the closest pass between them.
