Manually auto-filling a column by 3's.

My formula I am trying to use is "=IF($F$2=$F$2,SUM($W2,$W3,$W4))"...when I try to auto-fill down the column it increments by 1...so "=IF($F$2=$F$2,SUM($W3,$W4,$W5))"...."=IF($F$2=$F$2,SUM($W4,$W5,$W6))".
I need it to increment by 3's, without skipping rows. So I have a title for Y1, and Y2 is: "=IF($F$2=$F$2,SUM($W2,$W3,$W4))" Y3 should be: "=IF($F$2=$F$2,SUM($W5,$W6,$W7))" Y4: "=IF($F$2=$F$2,SUM($W8,$W9,$W10))"...etc.
I've been trying to figure this out for a while now. If anyone could help me out that would be awesome. =)