The reading on a gauge has almost nothing to do with what pressures the regulator is capable of handling or producing. A regulator can be fitted with any gauge a person wants whether or not the regulator is rated for those pressures.
"Daisy chaining" them together to get proper pressures may or may not accomplish what you're looking for depending on the regulators that you have. Generally a regulator is rated to handle a certain maximum input pressure and give a range of output pressures. Let's say for instance that your regulator is rated for a maximum input of 2000 psi and output pressures in the range of 0-300 psi as its gauge indicates. The regulator can produce anywhere from 0-300 psi. That said, it's not optimized for extreme precision anywhere in that range. So if you wanted better control you would want a regulator that was optimized for a smaller range (0-30psi is pretty common). This regulator with its very high maximum input pressure would be considered a primary regulator. The regulator with a smaller output range would be considered a secondary regulator and would generally have a much lower maximum input pressure (probably 150 or 200 psi, I'm not entirely sure...I come from compressed air not bottled gasses), and would need the pressure to be stepped down from bottle pressure to be used.
Now...on to the pieces that you have. The two greenish colored ones look to me to be primary regulators of some sort, whereas the bronzish colored one looks to me like a secondary regulator. That said, if it were me, I would look very carefully into exactly what those units are designed for both in terms of the gas they're designed to use and the pressure they're designed to handle. Also, they look pretty old, they may need repair depending on the condition of the seals/diaphrams/etc (assuming that's the style of regulator they are...like I said, I'm a compressed air guy, not a bottled gas guy).
For most scenarios, starting out you probably only need a primary regulator. As long as you can get it within a psi or two of where you want to be, you're probably ok. A secondary regulator is really handy when you wish to have multiple carbonation levels, or multiple serving pressures. Or for instance, if you wanted to be able to quick-carbonate something at a high pressure, while still being able to serve something else.
And last but not least....the disclaimer...these are just my opinions and the way I do things. Some people may disagree or do things differently. That's fine.
Hope some of that rambling helps.