TRY ANY ONE OF THE FOLLOWING :
1. =AVERAGE(IF(A1:A7<0, A1:A7))
This is an array formula and as such it must be commited with
Shift+Ctrl+<Enter

2. =SUMIF(A1:A7, "<0")/COUNTIF(A1:A7, "<0")

click yes below, if it works

"Help" wrote:

Help. I've calculated the average of three different cells in three different
rows that are not in sequential rows. The problem is that some of the cells
may contain a zero so the formula should only divide by two instead of three.
Anyone of the cells could be zero so you may have to divide by one, two or
three. Is there a formula???