What is the best way to turn EEG scans in to 3D images?
By 3D image, I assume that you want a projection of the topographic map onto a 3D model of the head. For an accurate resolution, you would need to have acquired the position of the EEG channels on the head as well as the shape of the participant's head. However, this kind of data is not often acquired.

The next best solution is to use an existing model of the channel position for your EEG system on a template head. The head plot function in EEGLAB can do that: http://sccn.ucsd.edu/eeglab/allfunctions/headplot.html

Transferring your files from SPM format to EEGLAB format should be quite straightforward. You can find solutions to this problem on both the EEGLAB and SPM mailing lists.