Servos are all interchangeable, no problems.
Just have to get the WIRING correct!
JR plug should fit into the Futaba's hole ("Futaba-J plug") but
it is no longer Keyed... it will go on both of 2 ways.
Check the Fut-J plug on the S3003, it has a ear/tag key on 1 side.
Plug it into the receiver, note how which way it goes in.
When you plug the JR plug in, make sure the wires going in
are coloured in the same sequence.
(nb: different companies may use different colours on the
signal and/or -ve wires, but always the +ve is red)