There is no time past the point of time where a traveler leaving the present and subsequently arriving in the past. If you can't retain old time lines or create new time lines. Sky net didn't send 3 Terminators back in time. 3 different Skynets sent 1 Terminator back in time. When the resistance sent Kyle back, they must have misunder stood that because they could see that there were Terminators in the past that it was their Skynet who sent them all back in time to the 20th century?