Error: ldap_simple_bind_s for <server3>:389 failed: Invalid credentials
because the argument to "credentials=" must be (to use the term of
slapd.conf(5)) a "simple password," unhashed. That is, do not put
"{MD5}blah," but rather put "secret" itself (which hashes to {MD5}*).
If this bothers you, switch to a SASL mechanism. You have the same issue
with <server2>, by the way.