You already have the flashes, just not sure why you went with those combinations. The 600EX-RT has radio capabilities, but the yongnuo's do not, you could have gone with a 580EXII for less $ and be able to get the exact same use in this case. No worries though since you can use RF transmission with your 600EX on camera to trigger the other two. So really all you would need is a setup like dpedro mentioned, two flash stands and two umbrellas, and you have a decent 2-3 light setup (3 if you use the one on camera as well).
That umbrella kit from b&h is a decent deal and a great start. Plenty good enough for modeling shoots.