> it seems to be easier for people to learn another language when they already know two or three.

Yes, that's empirically verified by multiple studies.

My pet theory about that is that a great deal of one's psychological sense of self is tied to one's ability to communicate with others. Learning a new language entails "letting go", to a great extent, of a linguistic sense of self. (Peter Hessler writes perceptively and humourously about this in one of his early books.) People who speak more than one language have either a) gone through this process (as adults) already, and can negotiate it more easily, or b) have (as dual-native speakers) a self-perception that is less-rigidly tied to a particular language context.

This is also why people who are highly articulate in their native tongue often progress more slowly than people who are not. I have more than once been humbled by someone who (natively) speaks what I'd (in my academic arrogance) judge to be "bad English" zooming ahead of me in foreign language acquisition. I'm concerned about being "correct", while they burble away unconcernedly and leave me far behind. Those experiences have been good for my character. :-)

See https://news.ycombinator.com/item?id=42077992, down-thread a bit, who makes this point better than I did.