Girls 1, girls 2, so the total number of girls is =

[C (bottom 3, top 1)*C (bottom 5, top 4)+C (bottom 3, top 2)*C (bottom 5, top 3)]*5!

=(3*5+3*5*4/2)* 120=5400

2、

One girl has already decided on one subject, so it is a combination arrangement of four out of seven.

Therefore, the total number of selection methods =7*6*5*4=840.

3、

A boy is fixed, so it's 4-7 combination arrangement multiplied by 4.

Therefore, the total number of selection methods =7*6*5*4*4=840*4=3360.

Note: Since there is no limit on the number of boys and girls in Questions 2 and 3, it is not necessary to consider boys and girls separately, but only the total number.