All three systems have their advantages and disadvantages. The Dolby system doesn't need a special screen and the filter can be attached to a standard Dolby Digital Cinema projector. This makes it an easy decision for cinema owners who are updating to Dolby Digital Cinema anyway, and also gives the cinema flexibility to show 3D films in auditoria that aren't normally dedicated to 3D. However, the glasses are expensive and it's believed that the filtering process limits the size of the screen to around 11.5 metres.
RealD is a more established system with cheaper glasses and can go larger (up to 18m). However, it has to use a 'ghostbusting' technique to counteract light leaking from one eye to another. This needs to be done when the film is mastered for distribution, which adds costs to the film production process, though RealD is working on a system that will work in real-time during projection. More seriously, RealD won't work in cinemas without a silver screen. The IMAX system, meanwhile, needs two projectors, which instantly ramps up costs for cinema owners. To counter this it benefits from a network of existing IMAX theatres, already fitted with high quality screens and sound systems, and also from a growing awareness of IMAX as a brand associated with high-end cinema experiences.
A fourth system exists - XpanD - which uses a pair of active shutter LCD glasses synchronised to the projected image. These alternately block off one eye then the next, allowing alternate frames for each eye to be combined to create a stereoscopic image. The system has been adopted in some cinemas across the US, Europe and Asia, but at the moment XpanD looks like a relatively small player in this competitive market.
As more cinemas convert or upgrade to 3D digital projection, more of us will get the chance to evaluate these systems. The technology used to produce 3D films isn't inextricably tied to the technology used to project them, so it's likely that major 3D releases such as Avatar will appear on all three. What's arguably more interesting is what happens when 3D follows widescreen and surround sound, leaves the cinema and comes home - we'll start talking about that next time.