The average marks scored by 8 boys is 72. If the scores of three girls are also included, the average marks increases by 3. The average marks scored by the three girls is:

Let's assume the average marks scored by the three girls is 'y'.

Total marks obtained by 3 girls = 3y

The average marks scored by 8 boys is 72.

Total marks obtained by 8 boys = $$72\times8$$ = 576

If the scores of three girls are also included, the average marks increases by 3.

$$\frac{576+3y}{8+3}\ =\ 72+3$$

$$\frac{576+3y}{11}\ =\ 75$$

576+3y = 825

3y = 825-576

3y = 249

y = 83

average marks scored by the three girls = y = 83