In an empty cell, type the formula =ISNUMBER(A3). If you get FALSE as a result, it means that A3 is not a number. It may look like a number, but in fact it is considered as a text by Calc, and therefore replaced by a 0 when doing calculations.
Note that you can hit CTRL+F8 to spot text/values/formulas depending on their color (text is black, values are green IIRC and formulas are blue).
Format > cell >numbers ad changed the selection on the left from text to numbers.