Hello everyone. Ive been advancing in my vipassana practice, but one doubt has come up: am I better off noticing all 3 characteristics while doing insight practice, or just one at a time? Which would you say is more productive in gaining insight? 

It can be useful to practice one at a time. But mostly just notice them as naturally/organically as you can.

​​​​​​​There's a sort of sweet spot where you can see them as they arise. One might be more obvious than others. Over time more and more of the three start to be included. But always just work with what you can see. 

Agree, rather than being neurotic about it, it's more like:

"wow, look how experience flows" (impermanent)
"wow, look how experience arises on its own) (not-self)
"interesting, i feel like I'm over here and the world is over there and there is a subtle struggle to percieve all this clearly, I'm not really getting it" (lacking, suffering)

and at any time one of these will be more easy to sense/feel/investigate.

My two cents - I always found it helpful to watch and observe the way experience plays out as a sit progresses, investigating the parts and pieces of its nature and letting the characteristics reveal themselves as insights. Pretending to know what to look for can be less effective. 

JMHO

It's worth keeping in mind that the three characteristics are tied to each other in multiple ways. That which is impermanent cannot make an enduring self and cannot be lastingly satisfactory. That which lacks a self cannot exist forever. That which is unsatisfactory cannot be the foundation of a self. And so on. We might even consider these three aspects of the same fundamental law of nature. So, in a way, it doesn't matter which of the 3Cs you notice at one particular time as, sooner or later, they will all become evident. 

They are interrelated, if you start watching one you will see the other two. For example impermanence of nice things is dukkha. If something is impermanent, you don't control it, it is not-self. 

So you can't really do dedicated sits for each. But if it seems simpler to do one, or there is one that is most relevant to you, then it's okay to try because you will get the other two anyway. You can change your focus from time to time - it doesn't matter.

I am most interested in ending dukkha so I tend to focus on dukkha (unpleasant thoughts and emotions) but I see how the ego-thinking (identity view) is behind most dukkha and how different aspects of the self image are involved (changing self image = impermanence/anatta), so I see the other two. Some people are more interested in not-self so they focus on that, but they will also see the other two. In the anapanasati sutta it says to observe impermanence, if you do that you will also see the other two.

Just sitting noticing your breath, not thinking about the three characteristics, you notice your breath is constantly in motion (impermanence), you get distracted by thoughts, you have emotions you don't want (you don't control you mind (not-self), and your mind produces dukkha). Arising and fading of thoughts emotions etc is impermanence too. I think it helps to be conscious about the three characteristics but if you meditate and quiet the mind so you can see what is happening - you will figure it out even if you aren't conscious of the three characteristics as the three characteristics - you see impermanence, not-self and dukkha - even if you don't have technical terms for them - and you will develop disenchantment and non-attachment.

I was told to just do a consistent  practice  of  mindfulness.  Don't look for the 3  characteristics.  The nanas will pop up on their own . Basically  what Chris M said.. I found it works.

What results have you had by trying both ways?
Have you bothered to try to answer you own question?
I'd start there.... Learn by doing. 
Or not
​​​​​​​~D