How should one formulate the intention for the Witr prayer, when performed as three separate rak'ahs, with a taslim in between each set of rak'ahs? Should one intend for two rak'ahs, and then intend for the concluding Witr with a single rak'ah?

The intention required is the intention for Witr for the first two rak'ahs, and also for the last rak'ah. It was said that for the rak'ah before the last, one should intend the night prayer or a مقدمة الوتر (prelude to Witr). Al-Nawawi, in Al-Majmu', said: If one performs Witr with more than one rak'ah, they should intend the entire set as Witr if it's with a single taslim. If it's with multiple taslims, they should intend each taslim as two rak'ahs of Witr. And it was said: for the rak'ah before the last, one should intend the night prayer, and it was said: the Sunnah of Witr, and it was said: a prelude to Witr. These different views concern preference, not conditionality, and the first view is the correct one.