On the P2 series, fast-blinking yellow means no signal, but a slow-blinking yellow means ATTI MODE.
ATTI mode on the P2 is when you are flying without the help of GPS, whether intentionally or not. It could be you have an inadequate number of satellites, or it could be you have deliberately chosen ATTI mode from your controller, meaning you are deliberately ignoring the satellites but they're there if you lose signal. On start up, ATTI mode is normal for at least the first few seconds. It might take a minute for the aircraft to find the six satellites needed for GPS. When it does, the rear lamps will blink green.
Not sure how this info transfers to the P3S, but if I remember correctly, the P3S uses straight GPS (6 sats), no glonass. See your P3S manual.
Good luck.