Calculation Cash flow

This proposal employs two helper columns which may be moved and/or hidden for aesthetic purposes.
I4:I10 are populated using: =IF(MOD(SUM(I3,1),B$14)=0,3,MOD(SUM(I3,1),B$14))
J4:J10 are populated using: =IF(I4>I3,J3,J$3+3*COUNTIFS(I$4:I4,3))
Note that J3 is populated with the min exponent (1.5)
G4:G10 are populated using: =(1+$G$2)^J4
H3:H10 are populated using: =IF(I3=1,(F2*G2+F3*G3)/2,F3*G3)
Let us know if you have any questions.