Can I use only one phase out of a triphasic cable?
If there is a provided neutral wire, for most countries it should be fine, 

Consequences:

If it was a large load, e.g. an outlet, you would need to make sure the load on each phase was reasonably balanced,

Having a lighting circuit on a different breaker to the rest of the house lighting is frowned at, as if that circuit trips, the light goes out, equally someone later on perhaps wanting to replace that light socket will switch off the lighting breaker, if they fail to test the light socket afterwards, they will have a nasty surprise.