Im thinking of buying 100m paracord, its a 3 mm thin, is it good choice, or should I order different thickness or even different material for rope?

I made my own trying to mimic the boleadoras that Gauchos use in Argentina by using thicker cordage and tying them onto hooks that I screwed into leather wrapped wooden balls. I lost it a while ago but over all the process took very little time (bar the leather), probably 10 - 15 minutes in one sitting. For that style in particular I feel like the majority of the time spent is probably on adding designs and making it look pretty as opposed to actually making the weapon. I've even seen a video of someone using paracord and hex nuts as bolas and I imagine that takes even less time to make.

Overall I think Bolas are one of the simplest weapons to make as basically everything can be however you want it, the length of rope, thickness, weight of the stones don't really matter as long as its weights on the end of a string. If you do want some guidance on length of string, while researching I read that some Gauchos leave one of the strings a bit longer than the other two, and that this is the one you hold as you swing it above your head. I made the two other ropes the length of my palm to my collarbone so they were pretty long and worked pretty well. In terms of weight as long as its not extremely light it should work fine, I even made some of tennis balls to throw at my friends.