I see that a truespeaker's caster level = their ranks in truespeak -3, does this mean that if a factotum found a lexicon he could learn those utterence based on the classes progression?
A truenamer’s caster level is equal to his ranks in Truenaming − 3. No one else’s caster level is calculated that way, and that includes the factotum. Unless the factotum is also a truenamer, the ranks in Truenaming don’t contribute to any caster level, and certainly don’t result in any ability to use utterances.
If the factotum is also a truenamer, through multiclassing, then his truenamer caster level is calculated the same as it is for any other truenamer: based on Truenaming ranks. The only thing that is slightly unusual is that factotum treats Truenaming as a class skill, making it easier to keep the skill maxed. This character’s ability to use utterances is still solely defined by his truenamer levels, just with more ranks in Truenaming than he’d otherwise have at his truenamer level.