Can I use only one Soundcore Life P3 earbud at a time?

Yes, when Bluetooth is connected you can use either earbud individually by placing the other one into the charging case — this is called Mono mode. Additionally, if one earbud runs out of battery and powers off, you can continue using the other one. During a call, placing one earbud in the case will automatically activate the microphone in the remaining earbud. (Page 5)