Your simplest solution would probably be a tree of power
Schottky diodes (or plain rectifiers, if you could stand
another half volt of drop) at the DC-DC input. Slightly
less lossy, but more complex & expensive, would be a rack
of power FETs performing the same function (select highest
input voltage, or some other logic (e.g. perhaps you prefer
solar input if it's producing -anything-, regardless of efficiency)
to select the source). Diodes can only do fairly dumb decisions.
Now, you might want to think about charge management
and stuff all sources into the battery, and then have a single
fixed battery-to-load converter.