Four different exoplanets have been found in systems with three suns. We've learned that a planet in a binary system (e.g. Tatooine) could actually have a stable enough orbit to be habitable. What about three suns?
Just like with binaries, it probably depends on how close the stars are together, how stable their orbits are, etc. Some systems can't form planets because the tidal forces prevent the dust in the system from coalescing into planets or even dwarf planets before it just gets pulled into a sun or two.
So far, likely because of detection bias, we've only found gas giants in trinary systems - not habitable by anything like us. We have found planets in the habitable zone of binary systems.
I think it's theoretically possible to have a stable enough orbit for life in a trinary system, but finding such a planet is going to continue to be hard - our current detection methods work much better for single stars.
One intriguing possibility for multiple star systems is habitable moons of gas giants, which depending on the orbital dynamics, may be more likely in multiples than around single stars. Imagine living on a world where you have one sun for part of the year and a different one for a different part...possibly resulting in some very interesting seasons indeed.
I should do something with that.