Alright a tinge bit more research and I think I have a bit more on all of this.
First, there
is (more than likely) an internal shuttle bus between the terminals, or at least NAIA-3 and NAIA-2 (the latter is walkable from NAIA-1). The catch? You have to be flying Philippine Airlines. Damn!
The bad news - traffic. Yeah, you already knew that. Most sources certainly recommend
at least 3 hours for the transit; on a good day that indicates arrival to onward departure. On a bad day, that might just cover your journey to the next terminal to make onward check-in.
The things which immediately have to come off the timer when considering feasibility of making it are:
- Arrival immigration. I don't know about NAIA-3, but I know that except for APEC diplomats and crew, there doesn't appear to be any fast track line. Balikbayan (Returning Filipinos) lines are supposed to be quicker, but that doesn't help you if you are not a Filipino(/a) or not related to one. NAIA-1 lines can be horrendous at times; hopefully NAIA-3 is faster.
- Security screening to get into the airport, i.e. the front door. I think this still exists at NAIA-1.
- Check-in, which usually closes 1 hour before departure, so that gets knocked off straight away from the timing, along with everything else.
- Payment of the airport tax (not sure if they are bundled in now) then outbound immigration and security screening. Immigration is the bottleneck here. If you don't make it out of here on time for your flight, the latter won't wait for you, period.
The shuttle which is available to people not flying PAL is supposed to come every 15-30 minutes, however there are reports indicating that the schedule is often way off (traffic, I guess), they often wait until the shuttle is full (or leave early if it fills up; some have bribed the driver to leave early). Many would rather just swallow it and go for a taxi, which would be more immediately available than the shuttle but can also be difficult to get, even if you avoid the already-illegal street taxis. Even then, one website reckons waiting to be able to score a taxi (coupon or yellow airport metered) could reach up to an hour (and the shuttle may not still have arrived yet).
The journey between NAIA-3 and NAIA-1, according to Google Maps, is 6.4 km by road. Google's estimate under "no traffic" conditions is that this journey would take about 30 minutes by vehicle. Another website I found gives a range of 30 minutes in good traffic, to 60 minutes in bad traffic. Driving between the terminals requires going through at least two roundabouts; you can imagine what these can be like. I used Street View to get an idea of the route, and it does appear that even though it's busy roads all the way, there is space to walk on the side of the roads, though not necessarily on both sides of the road (and the "walking side" can change from road to road or section to section), the amount of room can be quite limited, not necessarily smooth, and keep in mind that most of the road is "boulevard", i.e. there is an island in the middle dividing the two sets of lanes on the road, often there are tall plants or a fence which makes crossing the island impossible.
Also, entry into the grounds of NAIA-1 (maybe NAIA-3 too) requires passing through a manned security point. Basically they just have a quick look at the cars going through; not a demanding search by any means, unless you give off tell-tale signs. I imagine that people simply walking into NAIA-1 would be no problem, but worth noting.