How to structure a theorem with dependencies on lemmata in following sections
In general, it is a good idea to give your reader a "roadmap" of the approach that you will take early on.  You might even introduce the theorem itself early on, but leave the proof for later. 

Once you have established your roadmap, then you can proceed in order, building up to the theorem that the reader knows is coming (referencing you plan in the narrative that connects the mathematics).

Finally, when you have all three lemmas in place, you can return to the theorem, restate it, and state the simple proof drawing on all three lemmas.