Now there is a rather simple proof of III Lie theorem using the momentum map. It is in
Gijs M. Tuynman. An elementary proof of Lie’s Third Theorem. Publications de l’U.E.R. Mathematiques Pures et Appliquees, I.R.M.A. Univ. Lille 1 34 (1994), X1-X4,
see a copy in Google Cache (an original PDF seems unavailable online).
The author says in the end that almost the same proof is given in Gorbatsevich The construction of a simply connected Lie group with a given Lie algebra (here in Russian; an English version also exists).