How to safely down people?

I want to keep any attacking pirate alive and convert them into settlers. However, in the third raid, there are three guys and one of them dies every time. How can I down them without killing anyone? At least one of them keeps dying even when struck with clubs.

In vanilla, there is no surefire way to make sure you do not kill an enemy. However, there is a way to at least influence the odds of incapacitating an enemy.

Blunt weapons have a higher chance of incapacitating (as opposed to killing). A club will kill less people than a knife (assuming equal weapon damage).

Secondly, the difference between incapacitating and killing an enemy can also be a matter of how much damage you do to them. Use lower damage weapons, and you'll lower your chances of "overkill" which would otherwise turn a possible incapacitation into a guaranteed kill.

Thirdly, although this ties in with the blunt weapon argument: Rimworld models character health pretty in depth. If you've ever had a colonist who suffers from chronic pain, you'll notice that they faint quicker.
Every pawn has a certain pain threshold. When crossed, the pawn will lose consciousness.
So what you want to do is maximize pain in the enemy, while minimizing bodily harm. Blunt weapons, again, are the answer.

Alpha 17 has added the Wimp trait, which specifically lowers the pain threshold to a point where wimps will quickly faint.

And lastly, I also seem to remember that environmental elements (heat, cold, non-direct grenade blast) tend to incapacitate rather than kill. You could try trapping raiders in a room, slowly cooking them, and pulling them out after they lose consciousness but before they get killed.

None of these are guarantees. Globally speaking, Rimworld doesn't give you guarantees. Surgeries, animal training, crop harvesting chance, crop harvesting yield, food poisoning chance, furniture build quality, crafting build quality, ... almost everything relies on chance. Which can be influenced by traits and skills, but never fully controlled.

Alpha 17 even changed mining so that ore yield is effected by the pawn's mining skill. While not literally everything is a matter of chance (e.g. research), you can assume a vast majority of game play elements to be affected by chance in some way.

In vanilla, you can't. But you can install the No Death Randomness mod to remove the "67% chance for all non-colonist, non-prisoner pawns to die when they should only be getting downed instead".

Actually there is a way to get a nearly 100% down ratio. You need to build a heat trap!

Build a roofed zigzag maze as the way into your base with a bigger room at the end full of flamable material. Wooden stools or tainted appreal is my favorite.

When all the pawns of the raid entered the maze, seal in it off on both! sides with wall and torch the flammables inside. Then just wait until everyone goes down due to heatstroke, some weaker enemies will die though.

The enemies will start attacking the walls but won't be quick enough to escape. As soon as all are down deconstruct the wall and make the whole maze outdoor again so the heat will disappear in an instant and capture all your "loot".