Developmentally it seems that it's not uncommon to not like being alone at 3yo & needing that extra help to go to bed. It's definitely a "this too will pass", but a lot is that they are more independent at the same time as noticing that they need a trusted person more & throw in tiredness if they aren't napping etc.
From my own experiences, I'd have to say it's in many ways a problem with western society's expectation that they can just go to sleep on our desired routine, rather than a problem with a child's ability to go to sleep. Mentally they have huge leaps in the toddler/preschool times & the high amounts of mental processing going on mean they can't always switch off their brain like they did when younger & need the security of a parent to switch of one part of their brain & relax. Sometimes a longer routine works, for my eldest we found (for whatever reason) he'd settle to sleep in our bed & we'd carry him to his (didn't disrupt night sleep), whereas with my youngest, he was sharing with his brother & a shared bedtime & longer bedtime stories sorted it for us.