Well, the way I do it is one at a time. First, I create the biggest river all the way to it's end. I create the tapered stroke on that. I then start each of the additional rivers at the point I want them to join into the main one. I taper each of those as I create the path. (I also reduce the starting size so it doesn't look like you have 3 huge rivers becoming one little river.)

I hope that helps.