Total Work out of Three Bodies (Thermodynamics)

The problem is:

Having three bodies with thermal capacities (C) as sources of heat to a heat machine, what is the maximum work I can extract from this system, given that the bodies are at temperatures T₃, T₂ and T₁ (T₃>T₂>T₁), leaving them at an equal final temperature?

I tackled this problem by assuming that the amount of heat I can extract from each body is:

Q_i=C(T_i-T_f) i=1,2,3

And so:

W=ƩQ_i

Now, I'm not sure if I'm breaking the second law of thermodynamics by totally converting heat into work. This is where I'm kind of stuck.

So, if anyone can help me figure out how to solve this and other type-like problems, it'd be great.

Thank you :)