i think 3 is mostly just an arbitrary number. it is probably being selected because it's the number that planetfall used.
why did planetfall pick it? well, just my thinking on the topic, but it's a good number because it's large enough to provide a diversity of options while also small enough that it's a meaningful limit. would an enchantment limit of 2 work? it'd probably feel pretty bad, only getting to pick your 2 favorite enchantments for your units, and would pretty well devalue enchantment research if only 2 of them matter at a time. would an enchantment limit of 4 work? i'm not sure there's a compelling reason to say that 3 and 4 is where the dividing line is, but to me personally 4 does feel a little high if the goal is to curb potential power centration, and might be a bit much to keep track of.

