I need Help with an Excel Formula

Dhas160  you have 2 formulas in that cell.  Here I space it out to show:

=IF(AND(I3="X",N3+O3+P3+Q3>0,R3+S3<1),
    "SELL OFF FLOOR",
     ""),
 IF(R3+S3<3,
    "",
    IF(AND(R3+S3>3,R3+S3<6),
       "LOW STOCK",
       IF(R3+S3>5,"FAST DELIVERY!")
       )
     )

notice how the first IF statement ends on line 3 but then there is a comma and another IF statement starts on line 4.  I THINK you mean for that next IF statement to be the FALSE case  of the previous (i.e. cascading IF statements) like this:

=IF(AND(I3="X",N3+O3+P3+Q3>0,R3+S3<1),
    "SELL OFF FLOOR",
    IF(R3+S3<3,
       "",
       IF(AND(R3+S3>3,R3+S3<6),
          "LOW STOCK",
          IF(R3+S3>5,"FAST DELIVERY!")
          )
       )
    )

alternatively you could use IFS() statement instead:

=IFS(AND(I3="X",N3+O3+P3+Q3>0,R3+S3<1), "SELL OFF FLOOR",
     R3+S3<3,"",
     AND(R3+S3>3,R3+S3<6), "LOW STOCK",
     R3+S3>5,"FAST DELIVERY!",
     TRUE, "none of the above"
     )

the last thing I will point out is that I would add () around some of those conditionals because it is confusing if:

N3+O3+P3+Q3>0

means:

(N3+O3+P3+Q3) > 0

or 

N3+O3+P3+( Q3 > 0 )