I am very surprised too. A big pity that it is not possible to use the mix track for jack in. SD should remedy this.
Here is my little experience for those who starts to record near spaced pair and center mics. I am using a near spaced pair with a center mic only when recording stealth. My difficulties arise during mixing. I don't have good listening skills, it isn't easy for me to assess all positives and negatives when adding the middle microphone. Mostly I overhear the negatives at the first listening. Also, recording of third channel costs some effort, so I tend to use it. I don't want to throw it in the trash. But in the few cases I was recording in this configuration, I used only a single pair in the end. I didn't mix in the third microphone. Single near spaced pair sounded more natural itself and the third microphone was causing a little bit of comb filtering too. Sometimes, I did small adjustments of the stereo by Mid / Side manipulation. These are small changes as opposed to adding the middle microphone, but it can help a little bit too.
Gutbucket's advice is golden. I've never had a problem with center microphone when using bigger spacing and angling.