If you are just doing a pre-flight test e.g. in the house it's just indicating the GPS signal is not strong enough.
You may have enough satellites to get GPS lock at times, then as you lose one or two it goes to ATTI mode.

If you were flying outside when this happens it will drift with the wind and you need to be ready for much more of a manual flying experience.
For example once you initiate a movement in ATTI mode it will just keep sliding on for some distance unless you apply a gentle braking the other way.
It's worth practising in ATTI by flipping the switch on the corner.

It's very unusual for the P3* to lose satellite GPS because it can also use the Russian Glonas 
It happened quite often with the previous Phantom 2 (GPS only) because you would often have less than the required six satellites especially in less than ideal spot like near tall trees or buildings. 

*not entirely sure if the P3 standard has Glonas?