Front derailers are a pleasantly simple and straightforward part: you pull on a cable and the little metal cage pushes around your chain until it's in the right spot. You can look down and see it happening live, even. One spring balances things out. You could do the job with a stick or your shoe, but most would say that having a derailer is one of those worthwhile luxuries.

A lot of shifters, even today, will use a non-indexed mode for the front derailer because it's so simple. If that's the case you can use a double derailer on a triple crank if the tooth difference isn't above, say, fifteen. If your front shifting is indexed, it's best to use triple with triple and double with double, and try to match systems since it'll work a bit better, with less rubbing.

Unless your shifting system is super modern and fancy, don't worry too much about speed compatibility.