In a poem could any of the three adjectives below be used to describe a light (of a star, a lighthouse, a lamp, etc) that is weak and distant and which you can barely see:
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There are different kinds of poetry, of course.
Any of your three could be used. 'Faltering' is good. It suggests your star, lighthouse or lamp may be about to fail. 'Wavering' means the same but is less interesting. And it's rather a 'poetic' word and is therefore best avoided! The rare 'timorous' appears in such a famous poem by Robert Burns that it's inseparable from the word 'beastie' that follows it. So if you use 'timorous' your audience/readers/fanbase may start thinking about mice.
There are plenty of correct and appropriate words you could use. Ronald Sole's selection, for example, are all good and perfectly serviceable. Or you could find one that might not been used in this context before: one that surprises your readers/listeners/mum.
Here are some you shouldn't use without looking them up. Most of them are strictly-speaking wrong, but then who ever heard of fog rubbing its back against the window panes?): meagre, spineless, desultory, hypoglycemic, fainthearted, weakening, watery, degenerate, lapsing, tenuous, necrotic, illegible, enfeebled, frail, anaemic.
[In the US it's meager, not meagre. And in the US or perhaps the C21st it's anemic, not anaemic.]
While a poet is free to choose any words, none of yours really fits the context or image.
The words you suggest are generally employed in the description of people/animals and their attitudes/approaches/actions.
Such a light would usually be described as faint.
Other possible adjectives in this context might be indistinct, twinkling, glimmering or barely perceptible.