Hi,
a relay must be rated for the isolation voltage. Then there is no need to fear a failure connection between contact and coil.
There are regulations about air gap and creepage distances.
The regulations are difficult to read because the values depend on a lot of parameters.
Try to use 8mm of creepage distance on a PCB.
The wiring of the different voltage domains need to be separated ... or both (even the low voltage ones) need to be rated for the higher voltage.
A metal housing needs to be connected to earth ground.
I wonder why three wires, why not just the two relay connections?
Klaus