add condition to 'INDIRECT formula

[Jeff McCann]

I use the following formula to get A:1, A:2, A:3, etc.,
values placed in B:600, B:603, B:606, etc. (which allows
blank spaces for B601.B602, B604.B605, etc.:

=INDIRECT("A"&(ROW(3:3/3))

"then click F5 => 'special' => uncheck all
except 'constants'=> 'enter'=>'delete.'

Can you suggest how this same format can be accomplished
with use of a conditional, e.g., value for B:600 is A:1 if
A:1=x, (if not, 0); value B:603 =A:2 if A:2=x; etc.?

Your help is very much appreciated.

Jeff McCann