Formula for SUM if Not Blank and Display 3 If Less Than 3

Hi,

I am trying to set up a timesheet spread sheet that will sum if the cells aren't blank and return the answer as a minimum of 3. For the rows that are blank, I still want it to show 0. 

The basic formula to give the shift hours is =SUM(D15-C15-E15)*24).

The formula to only sum if not blank is =IF(C15="",0,(SUM(D15-C15-E15)*24)) which is somewhat redundant on it's own.

I've tried =IF(IF(C16="",0,(SUM(D16-C16-E16)*24))="<3",(D16-C16-E16)*24,3) but it doesn't give me 0 for rows that are blank.

Can anyone help me to get the right formula?

I have attached the file - the highlighted column in yellow is where the formula needs to go.

Basically, I want it to function as follows:

*If cell (C15) is blank, then return value of 0.00 and stop processing

*If cells are not blank, then SUM(D15-C15-E15)*24) then

*If sum returns value of less than 3, change value to 3.00