A single center omni will extend bass response of a pair of cardioids, and that's the first thing most folks think about when considering adding an omni. If only interested in that bass extension aspect, a low-pass filter on the omni will keep your near-spaced cardioid configuration from being affected above the low bass region and preserves the stereo qualities you're familiar with using a standard near-spaced stereo microphone configuration.
If the center omni is not low-pass filtered, imaging, recording timbre, and the ambient pickup will also be altered along with bass extension. That may be good or not good, depending. The recording will sound more ambient and reverberant by making the overall pickup more omnidirectional. Yet at the same time the stereo imaging will become less wide because of the introduction of additional common information to both channels. And their will be more comb-filtering due to the close proximity (without true coincidencce) of having three microphone locations interacting with each other, with smaller mics spacings between the center and side mics than what one had between just left and right in the two-channel config.
To offset the reduced imaging width and to reduce the comb-filtering and it's associated timbrel effects, increase the spacing and/or open up the angle between the Left and Right mics. How much depends on what center channel pickup pattern you've chosen and how much center channel level you are going to use (along with other things such as the particularities of the venue, sound system, and your setup location). I strongly recommend using more spacing whenever possible when adding a full range center mic between left and right microphones, partly because more angle between mics is not enough on it's own and is quite often not wanted anyway. In terms of just imaging, if its absolutely necessary to stick with a standard near-spaced two-channel mic configuration spacing between left and right microphones, a full 180 degrees between left and right mics is not only reasonable, it's not quite enough! If you can't space the left/right stereo pair mics more than you would for a 2-channel config, consider raising or lowering the omni by 2' or 3' on the same stand. That might sound like an odd suggestion, but the idea is to introduce sufficient distance between the three points in space to decorellate the reverberance sufficiently at medium and high frequencies (since we are using no low pass, remember?). Sufficient spacing reduces the comb-filtering timbrel affects and produces a more open sounding reverberant pickup.