Welcome to the Nikonians and congratulations on the new D5100. The short answer is yes, using the SB-700 on camera with two SB-600's as remotes will work using CLS as long as the SB-600's can see the infrared signals from the SB-700 master.

One other low cost option might be to buy 1 SB-700, 1 SB-600 and 1 SB-400 if money is a big issue. The SB-400 is not as powerful as the SB-600, but it really depends on what types of photography you want to pursue.

If you're experience with off-camera flash, you could look at non-Nikon flash units, but using CLS might not be possible.

Providing more information about your skill level, types of photography you wish to use the lighting for, etc. would allow forum members to answer your questions more precisely.

Good luck,

Larry

"Red is gray and yellow white, but we decide which is right
....and which is an illusion"

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