var1=123456
var2=RIGHT(ALLTRIM(STR(var1)),3)

var1=123456
var2=alltrim(str(substr(var1,1,3))) && Here You try to get first 3 symbols from var1 but substr wotks ONLY on string. You must change the sequence of the functions

var2=SUBSTR(ALLTRIM(STR(var1)),1,3)  && First 3 digits (in Your post you said You want the LAST ones. So first example gives you the last 3 digits