The loads should be across the supply, not in series.
You would be probably be better to have both DC/DC converters connected to the 5V supply.
The decision to use DC/DC converter or a low drop out regulator depends on what the average load is. If the load is very variable, you may be able to save power by having both a low drop out regulator and a DC/DC converter.
Yes, that is probably better, but googling this problem, I found out that it would be better to use resistor voltage divider or to use diodes (diode voltage drop).
Somebody suggested this:
5V Battery → Element 1: 5 V
5V Battery → 1N5818 → Element 2: 4,5 V
5V Battery → 1N4001 Diodes x 3 → Element 3: 3 V
Resistive voltage dividers are very inefficient. If you are using a solar powered battery, power is likely to be limited, so dividers are unlikely to be a good choice.
Diodes will be OK for some types of load, but the output voltage will not be very accurate.
The best choice depends on the power of the load. What is driven at 4.5 V and at 3 V ?
Element 1 - Raspberry Pi Model B - 5 V - 700 mA
Element 2 - Weather station base station - 3 x 1,5 V - very low consumption (alkaline batteries last 12 months min.)
Element 3 - Weather station sensors - 2 x 1,5 V - very low consumption (alkaline batteries last 18 months min.)
The diodes as voltage droppers would be a good choice there. Your 5 V supply will have to be regulated, so it won't vary much from 5 V. The parts that fun from the batteries will have to work on a range of voltages so the variation caused by temperature affecting the diodes won't be significant.