Well I was wrong because if the person enters a negative amount in say
a1 the total is negative and it won't allow me to continue entering
because the validation rule kicks in due to the total being less than
zero. Any Ideas?

Maybe a custom formula like this would help:
=OR($A$1="",$A$2="",$A$3="",$A$4=6)
It assumes that the three cells start all empty, they get entered in any
order, and the check gets activated when the last one is entered.