hello guys , i have to plans to do but have some problems

Plan number 1: In cell G1 I have the value of cell A1
and i want to drag by '3' so i can get in cell: G2 the value of A4
G3 the value of A7
G4 the value of A10 and so on

Plan number 2: In cell X1 I have the value of cell S1
and i want to drag in a way to get in cell : X4 the value of S2
X7 the value of S3
X10 the value of S4

can anyone help me , thank you for reading