The only reason that they send someone back is to kill Sarah Conners. If they don't do that then John is never born and they win by default. I don't believe that Skynet would ever lose to a human army. Computers just think and change and learn too fast. If AI ever really gained self-awareness, it would kill our race easily.
You've got the events out of order. It goes -
Timeline 1 -
Skynet created by humans, becomes self aware, sets off Judgement Day
Human survivors wage war against the machines
Under the leadership of John Connor, humans begin to win. Machines can't adapt/manufacture well enough
As a final hail mary, Skynet manages to create time travel device, sends back a T101 to kill Sarah Connor
Humans take time travel facility, send back Kyle Reese to protect Sarah Connor
Loop 1 -
T101 and Kyle Reese arrive in 1984. T101 is mostly destroyed, Kyle Reese is killed but has already impregnated Sarah Connor
Sarah Connor, now "knowing the future," drops off the grid to teach her son to be a great war leader
Cyberdyne Systems comes into posession of remains of destroyed T-101, makes huge technological advancements based upon them
Sarah Connor decides to change the future, attacks Cyberdyne systems herself, fails, is arrested and put in mental institution
John Connor sent to live with foster parents.
Cyberdyne Systems creates Skynet, it becomes self aware, Judgement Day happens
Under the leadership of John Connor, human survivors fight back and begin to win against Skynet
Skynet creates time travel facility, realizes it has already tried this once before and failed, this time sends T1000 to 1995 to kill John Connor as a child.
Humans capture time travel facility, realize a single human soldier can't hope to stop the super-advanced T1000 alone, instead reprogram T-101 to preserve John Connor's life and send it back to 1995.
Loop 2 -
T1000 and T-101 arrive in 1995.
John Connor's foster parents are killed, Sarah Connor is sprung from mental hospital.
Cyberdyne Systems is destroyed, Miles Dyson (Skynet's primary creator) is killed.
T1000 and T101 are both completely destroyed, along with the leftover remnants of the previous T-101.
Sarah Connor dies of leukemia.
John Connor starts living off the grid to avoid any further time travelling assassins.
General Brewster oversees the creation of Skynet and unmanned combat vehicles, T-1.
Skynet becomes self aware, Judgement Day happens.
John Connor leads humanity, humans start winning vs Skynet
Skynet constructs time travel facility as hail mary, realizes it has tried TWICE to kill John Connor and failed, instead targets his support structure, sends T-X to 2004 to assassinate as many high ranking human resistance officers as possible, including John Connor's wife, Kate Brewster.
Humans capture time travel facility, sends reprogrammed T-101 to 2004.
Loop 3-
T-X and T-101 arrive in 2004
Kate Brewster is saved, but Skynet attains self-awareness anyway.
T-101 sacrifices itself to destroy T-X, judgement day happens anyway.
(here ends Terminator 3, rise of the machines)
So... where we go from here.... is anybody's guess. But apparently it involves the human resistance sending Kyle Reese back to some period AFTER 1984, where Sarah Connor has already met the previous Kyle Reese and stopped the first T-101, thus retconning T2 and T3 and creating our first REAL paradox.