Stored procedure selecting from another scheme

When I try to run this stored procedure I get this error

PLS-00201: identifier schema2.table3 must be declared

I have looked everywhere and can't find the answer to this. I come from a T/SQL background so I am at a loss as to why this doesn't work.

CREATE OR REPLACE procedure schema1.select_ssstudent IS
begin

	select a.*,
		   b.*,
		   c.sitename
	from   schema1.table1 a,
		   schema1.table2 b,
		   schema2.table3 c
	where  a.permnum=b.permnum
	and	   substr(c.siteid,2,3) = trim(a.schoolnum)
	order by a.schoolnum, UPPER(a.lastname), UPPER(a.firstname);