Does a 3-5yr old understand separation?

When my son was 3, his mum and i separated. Hes now 5, never showed any signs of distress about the separation, i have him on weekends, he seems to be well adjusted, happy, friendly and well behaved. But he says he misses me. Which makes me wonder if he wonders why i cant be there all the time. Should i explain it to him? I didnt think that he'd be aware that we separated when he was 3, but according to all the articles ive read online, even toddlers are aware of a separation :/

Certainly not aware of the larger things happening, and you are right, he won't remember before, but the separation does affect him. But there is no reason he can't have a perfectly normal happy childhood.

Yes, discuss and speak with him. Ask him questions. Children FEEL, sense so much more than we are aware of.

I'd look into how to discuss why mommy and daddy don't live together by age ranges for such a discussion. Sometimes a simple 'daddy misses you, too' is suffice. Two of my three don't remember a before divorce life, so they have randomly claimed at 11 and 10. They were 4 and 3. My 14 year old has a choppy recollection, at best. And at this stage it's really more of a non issue as far as their feelings because they don't make it an issue. Living nearby yet seperate is their norm. Their father works many days and sees them but once a week for a couple of hours, at that. That might be something they will address internally over time. As it's on him not me.
At these ages, letting kids be kids without worrying/fretting over adult stuff is more important.