3. In the figure, point O is the center of the circle and OA = 3. The angle ACB = 60 degrees. Find CO.

This problem involves a circle and a triangle. The angle ACB is an inscribed angle subtended by the arc AB. The central angle subtended by the same arc is AOB. The relationship between an inscribed angle and a central angle subtending the same arc is that the central angle is twice the inscribed angle.

However, the provided image and question for problem 3 do not offer enough information to definitively determine the length of CO. We are given that OA = 3, which is the radius of the circle. Point C is outside the circle, and angles ACB and a point labeled '?' are given, but without further relationships or lengths, CO cannot be calculated solely from the provided information and diagram.

To solve for CO, additional information would be needed, such as the length of AC, BC, or the measure of angle AOC or BOC, or if triangle ACB is a right triangle.

Based on the visible information, the length of CO cannot be determined.