I went through all this with my P3S ..... I eventually had her reaching 5km out from home .... but that then proved my downfall when wind proved too much for her to get home and she went for a swim. She now sits in my workshop dead as a dodo.
First is to get rid of the stock antenna .. both 5.8 (external) and 2.4 (internal). Replace with such as the ARGTEK or ITELITE. ARGTEK is easier to add the boosters in my opinion.
I bought two 2.4Ghz bi-directional boosters of 2W each ... 5V powered.
I bought a single 5.8Ghx booster also of 2W.... powered by 5 - 18V
The boosters were then added between the mounts and the antenna ... with power coming of a LiFe 2S pack of 6.6V. The 2.4Ghz were via a 5V BEC to reduce to required level. The 5.8 powered direct of the LiFe battery. No need for a switch as I used JST connector that was easily unplugged.
Here's an average 4.5km flight ... its the screen recording so you can read the numbers yourself !
OK ... for a simpler solution not requiring boosters or power ... I glued two Windsurfers together and slid over the ARGTEK centre 5.8 antenna leaving the 2.4's as is. I had over 4kms without any trouble.
Again screen numbers do not lie ...
Nigel