It's generally a good idea to provide an "after" view so we can see what the end result "should" look like... it's not very clear.

If the intention is to highlight any days where at that point fewer than 3 consecutive days have been worked then in terms of a possible CF rule:

=IF($B6="",0,IF(C6<>1,1,IF(COLUMNS($C6:C6)>=3,(SUM(OFFSET(C6,,,1,-3))<>3)+0,0)))
The IFs are just used to ensure only nec. calcs are performed

(If you have a row starting with 1,1 neither would be highlighted as we don't know the run at the end of the prior month)