What visa are you getting for three months ? You'll need to attend immigration as you only get 30 days on arrival, then a 29 day extension waiver can be applied for, then there is more plasterwork needed after that for further extensions.
Supposed to be able to do it online but.... π it does't work for me
https://e-services.immigration.gov.ph/
As to your other question about a one way ticket, I have been asked once in many trips to Thailand, I lied and said I was catching the bus to Cambodia, that seemed to satisfy the counter staff, presumably because I had an answer ? I have never been asked at immigration, only by airline counter staff upon departure from Australia. Of course that wont work for The Philippines and it does say on the front page of the immigration web site
https://evisa.gov.ph/
though the last two times I have only had one way tickets
You're also supposed to register with their visitor app, last time airport staff were asking arrivals if they had and pulling people aside to go do it before they get to immigration if they hadn't. I had but there was no checking at immigration. That was last year though.
https://etravel.gov.ph/