When dealing with lap only belts, there is an argument that a booster seat increases risk because it places the child higher and slightly more forward which increases their risk of impacting on other items in the vehicle, which is increased with a lap belt because nothing is restraining the upper body. This is the reasoning that all booster seats state that they must only be used with a 3-point seatbelt, not a lap only belt. However the evidence doesn't give a clear consensus either way. Booster seats help position a seatbelt better over the child's hips/thighs rather than the soft abdomen, so there's an argument to use them together for that reason.
Booster seats (even backless) are worth using with a 3-point seatbelt.
I don't think there are any ERF seats on the market any more which can be fitted with just a lap belt (+ tethers) - there used to be. I have a feeling Nachfolger World might be able to be fitted with a lap belt and brace on the seat in front in a pinch, (the manual states that this is only to be done on a dashboard) but not 100% sure, plus it's expensive and only up to 18kg.
You could take something, and if there is a 3-point belt then use it, if there is only a lap belt, then use the lap belt as better than nothing, even for a 3yo. The compartmentalisation of a coach, with the smallish gaps between seats, is also a positive if there is an accident, and so is the physical size and weight of the vehicle - in any collision, accident force is much more impactful on the smaller/lighter vehicle. BTW, if you end up with a child on an adult's lap, never share the seatbelt. The adult should wear the seatbelt and the child should not.
You could consider Tinyseats 2 if there is a possibility of having access to isofix, this would allow possibly both children to RF and can later be used as a high back booster, however again it's a pricey option.
Borrowing the HBBs from the hire company sounds like a reasonable approach. If you wanted to take your own, there are folding HBBs available, the Cybex one mentioned plus Maxi Cosi Tanza i-size and a couple of others by Becool/Recaro. Or you could get something cheap and basic, like Graco Junior Maxi (the R129/i-size version is much better than the old version) and separate the backrest and seat parts for transport.
But TBH, the Maxi Cosi one didn't get a fantastic ADAC score - it was definitely better than nothing, but not as good as you'd hope - and the others haven't been tested (apart from Axkid Up which is extroadinarily expensive too). A HBB is mainly a belt positioning device - the condition and model does make a difference, in that a poorly designed one will often route the belt in a less optimal way or be harder to adjust correctly, but cheap/old doesn't necessarily mean it will be badly designed. Some of the cheaper models are absolutely fine and do a fine job. There is even research showing the side wings are much less important than previously thought because in real life crashes, drivers tend to brake in anticipation of impact, which moves passengers forward and outside the reach of the side wings. So the standard crash test to test side impact protection is not necessarily realistic for forward facing seats/HBBs in particular.
There is a vest sold in USA called Ride Safer Travel Vest. This can be used in combination with a 3-point seatbelt, it doesn't meet European car seat standards, but that wouldn't necessarily matter as the three transport options you describe are usually exempt from car seat requirements anyway. I don't really understand how it works to redistribute crash forces, but the US regulation has approved it, and if you could get a legitimate version of this imported, it would be more portable and might be better than nothing. Be careful of copycat versions though as these have no regulation.
Try to bear in mind that people survived car accidents even before car seats/seatbelts were a thing, and that an accident is very unlikely to happen. I definitely understand your reasoning and agree that in an ideal scenario, more protection is better but try not to let worry about this override your joy of the holiday. Do what you can, but also try to make peace with understanding that the risk is low to begin with. Sometimes catering to anxiety like this makes it worse which can then become more restrictive over time. So just to say look after yourself with it as well 