Your COUNTIF
will work for <30, although I'd write it slightly differently
=COUNTIF('Open Sales Orders'!AH2:AH484,"<30")
and you can do similar for the largest values
=COUNTIF('Open Sales Orders'!AH2:AH484,">=90")
for the between
conditions use COUNTIFS (with an "S") which allows multiple conditions, e.g.
=COUNTIFS('Open Sales Orders'!AH2:AH484,">=30",'Open Sales Orders'!AH2:AH484,"<60")
and
=COUNTIFS('Open Sales Orders'!AH2:AH484,">=60",'Open Sales Orders'!AH2:AH484,"<90")