In Catullus 1,
Iam tum, cum ausus es unus Italorum
omne aevum tribus explicare chartis
doctis, Iuppiter, et laboriosis.
The only ways to justify 'omne' are:
These are the only options that make grammatical sense to me.
Based on what Catullus is trying to say, it makes sense to make 'omne' agree with 'aevum', even though it doesn't.
Why is 'omne' in the ablative?
It's a regular neuter accusative singular form, modifying aevum:
...you alone of the Italians ventured to unroll all of time...
The ablative singular form would be omnī.
'Alone (out) of all the Italians' would be something like unus ex omnibus Italis or unus omnium Italorum. An ablative singular wouldn't work here (even if the form were ablative singular); and, although we say in English 'all of the Italians,' Latin tends not to use a genitive with words like omnis.
'All three learned scrolls' would be omnibus tribus cartis doctis. Again, an ablative singular wouldn't work, because there's no way to make it agree with the rest of the phrase (tribus cartis doctis), which is ablative plural.
