My flight from Osaka to Manila is from 13:45-17:00 on Jetstar airline. I am planning to book another flight from Manila to Tacloban the same day at 18:15. It's the last flight going to tacloban on Cebu Pacific airline. If I book it, will I be able to make it from terminal 1 to terminal 3? Should I book it or just book the next day earliest flight?
Book a flight the next day. Taking the 1815 flight gives you 75 minutes to get off the plane, pass through immigration, collect your bags, check in, pass through security and get on the next flight. Also note that you will have to re-check your bags before check-in closes, which is typically somewhere between 30 and 60 minutes before departure. That could leave you very little time.
I don't know Manila airport but I wouldn't try to do that anywhere in the world. If your incoming flight is late, you have even less time. Further, because the two flights are on separate tickets, Cebu Pacific would have no requirement to put you on a later flight if you missed the connection: it's your responsibility to get to the airport on time and the airline doesn't care if you're coming from Osaka or downtown Manila.
Based on my experience from flying through Manila many times in the last 10 years you are really cutting it tight.
Because you need to go through immigrations and also fetch your bags, re check in for your transit most likely etc
I am a betting man and I would lay 5-1 that you would miss the fight meaning I think you have less than 20% chance of making it, a lot of things have to go really right for you to have a chance.