B>A>C

A>B>C, the predicative part expressed with an adjective 'human' is more stylistically correct. Next one is B, the predicative part expressed with an infinitive 'to err'. These two variations make a bit different senses, depending on the last emphasized words - 'to err' & 'human'

champmag wrote:B>A>C

This is what I'd have gone with using your actual sentences, with the caveat that To err is human as a phrase comes to us from the poetry of Alexander Pope in the 18th century (and as such is not the best model for modern prose usage, though that phrase itself is accepted as normal). Erring also feels as though it might have some complications as a gerund. For example, I would choose Playing is fun over To play is fun; I am not so well-versed in linguistics that I could tell you whether there is a deeper structure there, but I have a vague sense that it might have something to do with the verb as an actual intentional activity, if that makes sense. I would expect:

Sleeping is important to me on the weekends or Sleep is important to me on the weekends
over
To sleep is important to me on the weekends (in part because the Shakespearean To sleep, perchance to dream from Hamlet might lead the reader to expect a second infinitive)

but

Digestion is an important biological process
over
To digest is an important biological process
or
Digesting is an important biological process