Hector 3 starts with one objective: prevent Hector 2 from luring Hector 1 to the time machine. It would be fatal if he were to succeed, because he would undo his own existence pushing everything back to the lost original history in which Hector 1 wound up in the time machine--an infinity loop. However, Hector 3 has not thought that far ahead.
At first he intends to beat Hector 2 to the girl, but the gate is locked against him and by the time he can get a vehicle with a gate key Hector 2 is already ahead of him on the road. He next thinks that he can manage this if he crashes the pickup into the white car, injuring Hector 2 and so keeping him from pursuing the girl, perhaps again intervening such that he meets the girl instead, and sends her to safety. This also fails, because the girl finds Hector 2 and not Hector 3, and so Hector 2 starts manipulating her into his plan while Hector 3 hangs unconscious nearby in the inverted pickup truck.
By the time Hector 3 awakens, Hector 1 is already headed for the compound with Hector 2 in pursuit. Upset that his plan to prevent Hector 2 from catching the girl failed, he radios the researcher and tells him not to allow Hector 2 to become Hector 3. It is unclear why Hector 2, who is carrying the walkie and walking in the quiet woods listening for some sound from Hector 1, does not hear this, but apparently he does not.
Fortuitously, when Hector 3 goes into the woods the girl happens upon him. Her scream diverts Hector 2 from his pursuit of Hector 1, but Hector 3 is right that they are hidden where Hector 2 won't look. In this timeline, Hector 2 has been bandaged from the moment she saw him, so she does not recognize Hector 3 and is easily persuaded by the badly injured man that they might both be fleeing from the same attacker. She eventually persuades him to come with her to the nearby house--his house--for help, and although he attempts to dissuade her, she is determined and he is injured, having twice been in the same automobile accident and sustained head injuries both times.
When the girl leaves him in the kitchen and Clara appears returning from the store, he gets his idea. He is going to attempt to lure Hector 2 away from Clara, using the girl and the ladder as decoys. He probably hopes that Hector 2 will hear the girl, then see the ladder and think that the girl got from the attic to the roof and down the ladder. However, he is also hedging his bets: he disguises the girl to look as much like Clara as he can manage in a few minutes, so that if the girl falls Hector 2 will think it was Clara. He then moves Clara to a safe location, settling next to her to establish his own alibi for the events to come.
The plan works quite well. The girl falls to her death, and Hector 2 mistakes her for Clara. At that moment the walkie-talkie reminds him that Hector 1 needs to be chased into the tank, and he grabs the car keys and heads for the compound. After that he, in his turn, becomes Hector 3.