The main thing that takes up data allowance from Graal is downloading the custom graphics (heads, bodies) from the players that appear on your screen. Every time you encounter a graphic file for the first time, that file is downloaded and stored in Graal's cache for quick loading the next time it is encountered.
If you have played Graal a lot on your device without deleting and reinstalling the app (because reinstalling resets the cache), then you will probably have a lot of the graphics cached, so playing Graal on 3G will not consume a lot of data. The game does download data about player positions and actions, but this is actually very small and won't affect your data allowance significantly.
If you have iOS 5 or above, you can check how much data has been cached by Graal by going into Settings > General > Usage > GraalOnline Classic. The "Documents & Data" section shows the cache size. A larger cache here would mean more graphics already downloaded (hopefully you did this over Wi-Fi), and hence less will be downloaded when you use 3G.
Another way to check usage is to open Settings > General > Usage > Mobile Data (or "Cellular Data", depending on your device's language region settings) and see how much data was downloaded before playing Graal on 3G, and then compare that number with after playing Graal.