O/k, +12v battery input only, and the output needs to be +8v max.
So it looks like a purely step down requirement. A healthy battery should not drop below 8v during cranking. If it does, the solution is a new battery, not a better dc/dc supply. The main thing during engine cranking is starting the engine, and as long as the ignition still works, it usually does not matter if other things do not.
Its also quite possible that in a vehicle, an isolated supply may not be a requirement, so a simple buck regulator may be all that is needed for the main +8v output.
The two lower voltage outputs can best be served with linear regulators, working off the main +8v output, as the power requirement is fairly low, and efficiency will still be pretty good.
It will need some high voltage transient protection at the input, and be safe for load dump input voltages, but a buck regulator should be able to handle that with ease.
Many of the control chip suppliers provide sample circuits that produce high current outputs at +5v.
A slight tweak should see that increased to +8v without a serious redesign.