If you have a 3GB int+27GB local cap, then unfortunately, there is no way to split the international and local bandwidth.
This kind of internet account is really:
3GB combined bandwidth with a soft cap of 27GB local only.
What this means is that this account will just start automatically with your 3GB combined bandwidth (and these 3GB will also get used if you access local sites)
Once the 3GB is up, you will then be transferred to the LOCAL only bandwidth.
So in theory, you are FORCED to do your international at the beginning of the month, and then once that is used up, openbrowse will go ontop of your local bandwidth to give you browsing abilities.
The ONLY way to do what you want to do is to buy TWO separate internet accounts.
1) An international 3GB account (IS semi-shaped or SAIX shaped)
2) A local only account (IS local only 30GB for example)
If you now have 2 separate accounts, you can configure your router in "bridge" mode, and just switch between "local only+openbrowse" and "international" account at your will, thus only using the international bandwidth when you want... (if u don't know what bridge mode is, let me know and I will try guide you)
Hope I was clear enough.