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I have a three-phase supply voltage, between them is 415 V.
This answer is based on the presumption that you are in India with a 3 x 415 V ~ 50 Hz utility supply.
Hence three-phase 415 V, single-phase 415 V and single-phase 240 V would be readily available.
There would be no need for a 415/230 V step-down transformer or earthing.
Can I connect a 415/230 V step-down transformer between any two phases and use the output of the transformer as phase and neutral?
You can but, it's a dangerous situation you have created without both the neutral being earthed and, an earth wire distributed with the live and neutral to where you want it to go. Use of earth leakage/residual current devices are also required along with a suitable breaker for the cable type you are wanting to use. Your earth needs to be good i.e. use a decent earth rod.
Or should I earth the neutral and use it?
See the above.
Or should I use a transformer between one phase and earth?
No, that's likely to be against your local regulations.
If you have 3 phases and 415V between them (Europe and most of Asia consumer power standard), chances are that you also have a neutral and 230V between each phase and neutral so no transformer is needed to get 230V.
Your idea of using a step-down transformer 415 to 230 is sometimes used for different purposes:
Getting 230V supply that has neither side connected to Earth and neither called Neutral. This is used for lab bench supply or for dealing with older equipment.
Or, maybe, the middle of the secondary connected to Earth to get 2x115V split-phase in order to mimmic the American mains.
Both 1. and 2. can be used as well with 230-to-230 transformer, but using a higher primary has its advantages, esp. when dealing with significant power levels.
You should not use protective earth to power anything.
The secondary of a transformer is "floating":
not related to any of the phases, neutral or protective earth.
You can use it, loading the network in an unbalanced way (hard not to with a single phase load), and there will be a phase difference of at least 30° in each of the phase wires even with a resistive load:
Power factor ≤ 0.866.
It does not give you a neutral potential.
If you have access to a neutral potential as well as to at least one phase of a three-phase network with a line-to-line voltage of 415 V nominal, you could
There are grounding transformers capable to get a neutral potential from all three phases of an "ungrounded" symmetrical three-phase system.
Answer to revision 1:
Using a transformer to get a neutral potential from just two of the phases of a network at least somewhat commonly used for power distribution will work only when those are the phases in a "high leg" network that are not the high leg.
And in that case, the transformer needs to be 1:1.
