① If only one Party B array is arranged, the redundant number is 8×8=64 (persons);

② Arrange a solid C-shaped square, and the insufficient number is: 8×8=64 (people).

(3) Assuming that the square C is a solid square, the number of people in the square B is 8×8+8×8= 128 (people), and the square is extended by one layer, with 2 people on each side. There are 4 more people in Party C than in Party B, and the number of floors in Party C is 4÷2=2 (floors), so 2 floors need to be added.

Solution:

①

Suppose C-square is a solid square.

Then the number of people in the outermost layer of the square matrix =8×8+8×8+2×4)÷2=68 (people)

②

Number of people on the outermost side of Party C =:68÷4+ 1= 18 (person)

③

Total number of people in the hollow square =18×18-8× 8 = 324-64 = 260 (people).

A: In the fifth grade, 260 people participated in the broadcast gymnastics competition.