Can anyone tell me what RLM # this is: XF-59:3+XF-64:1?

The Germans used both types, but the brown one was more common. I would suggest doing some research on your particular aircraft. If you can't find any, or you don't care, you could paint it whichever suits your taste. I prefer the yellow one since it is less common.

2:1 would be the brown one.
3:1 would be the yellow one.

But is RLM 79 what I need? The Tamiya kit says a 3:1 ratio and from what I can find, RLM 79 is a 2:1 ratio. Since XR-59 is desert yellow, I'm guessing that I would want the more yellow one. I'm painting a Bf-109E4 in desert camo. Perhaps I should have asked what Germans used on their desert camo schemes instead of what Tamiya is asking for.

There are two types of RLM 79: a yellow one, and a brown one. Model Master produces the yellow one, and Humbrol and GS produce the brown one.