Again some problem, Try this way,
Right click and Insert Script Object
When you add script object it should look like below,
Then call it just like below,
var retVal = jsGlobal.abweichungSummen(5,2,2);
Don't know why you are using == (double equal to).
Also make sure that you are having valu in your parametersG_SUMME_NETTO_GES, SUM_AKT_MON, SUM_KOR_MON.
Hope this helps..