Agreed that three cards is the safer bet and probably the best way way to go. If you weren't mixing down to 2-ch stereo until later I'd suggest an omni in the middle, as i suspect that would make for a better addition to a standard NOS pair, providing low-frequency extension and weight on the low-end and some openness up top without collapsing the stereo image as much in the cardioid "presence region" when blended in at the same level as a cardioid.
From my own experience with dialing in the level of the center vs the L/R pair afterward by ear- it's very satisfying to do, but usually a bit of fidgety thing to get perfect, so probably best to go with a safe, somewhat lower level from the center mic. Without playing around with it via trial runs in that venue or one similar it'll be difficult to nail perfectly, and even then the optimal mix will vary from performance to performance. Too much center will make the resulting recording overly mono and sort of boring, while none at all will just revert to standard NOS (or whatever config you setup the outer pair as), so the safer bet will be to use a bit less center "blend" than you might otherwise be tempted to use. Obviously nothing you don't already know here.
Ideally, the presence of that center microphone allows you to space the L/R mics farther apart, or angle them more widely, or a bit of both, but that's counter to the needs of the situation you describe, so just plan to use less of the center. If you were in a situation where you were able to space them or angle them more widely, you could expect to be abler to use more level from the center mic. For example, the 3 supercardioid channels that make up the L/C/R center portion of my current arrangement usually get mixed down to stereo with C at around the same level as the L/R pair. But while the angle between the L and R mics is the same as NOS (90 degrees), the spacing between them is around twice NOS spacing, which accommodates the center microphone while keeping the L/R pair angled forward as much as I like. Without the center channel, that wider arrangement would produce a weak center, but with the center mic in use that change allows more room for it in the mix.
But you might consider widening the L/R angle a bit, even if that will point the L/R pair somewhat farther off-axis from the PA. Doing that won't make the array any wider physically so it wont impact sightline concerns, and a wider angle is likely to be less of a potential problem outdoors than it might indoors. A wider ORTF angle of 120 degrees while retaining the NOS spacing of 12" would be a safe bet and allow the 3-mic arrangement to differentiate itself a bit more in comparison to the typical 2-channel mic setups. But just an idea, you know the venue and the sound you like the best.
