First of all, the answer: it can be covered. And the diameter of the disc reaches 12.89CM, so it can be covered.

1. If a circle is to cover three squares with side length A, when the squares are placed in zigzag, it will cover the largest side length. Let the side length of a square be a and place it in the shape of a "needle". Then the circle should be inscribed with two points above and two points below.

2. The center of the circle is on the vertical line in the middle of the "needle" shape, a little below the intersection of three squares. So the distance from the center of the circle to four o'clock can be equal.

3. Let the distance from the center of the circle to the horizontal line at the top of the pin shape be X and the distance from the horizontal line at the bottom be Y, then x+y=2a, and according to the Pythagorean law xx+(a/2)(a/2)=yy+aa, X =19a16 can be obtained; y= 13a/ 16

4. So the radius of the circle under the root sign is: (aa+YY) =1.289a.

5, so 1.289*5=6.445, so it can be covered as long as the diameter reaches 6.445*2= 12.89.

It's so hard ~ ~ ~ ~ ~