I think that it’s hard to throw a top 3 supplements up without the specific details / context. Meaning, it’s hard because people are coming into the paleo lifestyle from different places and with different profiles… plus people jump into the lifestyle at different rates and via different approaches.
That said, in my very limited experience (and I’m not a Dr), I’ve noticed that when a person jumps into this lifestyle they tend to do two things: (1) They tend to go overboard with the low carb meme (particularly fruit consumption), and (2) They tend to hit the gym / workout way too hard (when they also tend to be the most out of shape or are still tweaking their fitness routines).
Assuming that the individual in question is starting out by taking their food intake and sources seriously, is reading up on the subject matter, and is in the process of tweaking the lifestyle to suit such persons specific needs, my top 3 supplements for the initial / short term are (in no particular order):
Vit D3: most folks are deficient, and you can’t consume it enough via food, etc etc etc (all of the reasons stated herein);
Vit C: if your diet consisted of a lot of sugar, then there is a high probability that you are deficient. Additionally, people tend to immediately drop to virtually zero their consumption of fruit (which for most folks is their only source of the vitamin) and haven’t really picked up eating proper quantities of Vit C veggies (as they tweak the diet or transition). It’s a hugely important antioxidant (particularly if you’re working out, in excess or otherwise), in amino acid metabolism (collagen), absorption of iron, etc etc etc.
Magnesium: Quite a few folks are deficient (particularly due to excessive sugar consumption), and once they drop flour (and as they tweak their new diets) they tend to magnify this because it used to be a source of it under poor diets. Magnesium is key in atp metabolism, protein synthesis, muscle relaxation, cardiovascular health, insulin resistance and production, calcium absorption, etc etc etc.
The last two have laxative properties in excess. Spread the dosage throughout meals, I use concentrace mineral drops to control /spread the dosage as appropriate.
It’s important to note that I did not throw up O3 supplementation because it tends to be abused, harder to really shoot for optimal “ratios”, and quality (or ability to control it) of the products (oxidation). I recommend a can of atlantic sardines every other day instead – O3 laden with tons of other minerals, nutrients, and vitamins to boot.