Even if Transformers franchise is critically a mess, the analogy with the Transformers franchise can be used to guauge IM3 potential reception abroad.
1) TF2 did disappoint like IM2 desappointed from their respective first movie, both reaching respectively 310+ millions for IM2 and 410+ millions abroad for TF2.
2) Both were in 2D.
3) Then they decide to include 3D for the third movie.
I say if a mediocre TF3 could rise its foreign take from 52 % for TF2 to 68 % for TF3 just by the inclusion of 3D, 4 years ago, reaching 770 millions, i say it bodes well for IM3 on the foreign front using the same tool for a popular franchise.
I therefore don't see why,the inclusion of 3D alone wouldn't push IM3 in the same way it did for TF3, and this without even including the new consumers from TA. It did 50% in the foreign front with IM2. It could very well behave like TF3 and rise this ratio abroad putting it at 66%. No matter what this ratio will rise anyway because of 3D and because of TA. The only unknown element, is how much it could rise.
Since it is nearly guaranteed to reach $350 millions+ domestic, a 66% share representing the foreign box office will put it at $700 millions abroad. Personally that doesn't seem so foolish or untattainable to me.