Hello guys, I need help.

I have a spreadsheet that i need to ad these formulas from columns F-
Dy. Starting with cell F3, F4, F5 and skip F6 and start over at F7,
F8, F9. in other words add these 3 formulas starting with cell
F3,F4,F5, skip F6,F7,F8, skip.... going all the way down to cell F873.
i need this with all the columns and chage the formula letter with the
matching column

the formulas:

=SUM(5.15*80)+(5.15*1.5*40) (F3)
=SUM(F3-F2) (F4)
=SUM(F2/120)*(40*0.5) (F5)
BLANK BLANK (F6)
=SUM(5.15*80)+(5.15*1.5*40) (F7)
=SUM(F7-F6) (F8)
=SUM(F6/120)*(40*0.5) (F9)
BLANK BLANK (F10)
=SUM(5.15*80)+(5.15*1.5*40) (F11)
=SUM(F11-F10) (F12)
=SUM(F10/120)*(40*0.5) (F13)