Arithmetic sequence follows the order a,a+d,a+2d,a+3d….

First term is a , a= 3

Second term a+d= 7 . substitute a here to find d

3+d=7, d=4

Third term a+2d=11

Fourth term a+3d=15

We need to fifth,sixth and seventh term

Fifth term a+4d= 3+4*4 = 19

Sixth term a+5d = 3+5*4= 23

Seventh term a+6d=3+6*4= 27

The next three terms are 19,23,27