But why did they need 3 sheets in 2 pieces?
Actually, 3-sheets were originally in 3 pieces. The first presses could only accommodate a 28 x 42 sheet (some places printed that size, and others 27 x 41). When larger sizes were created, they were just multiples of that, so 3-sheets printed on 3 one-sheet sized pieces (laid on their sides, to measure 41 x 81), 6-sheets printed on 6 one-sheet sized pieces, and 24-sheets printed on 24 one-sheet sized pieces. This was true into the late 1920s. In England, the "sheet" was 30 x 40, so English 3-sheets measured 40 x 90 or so.
In the late 1920s, presses that could print larger "two-sheet sized" pieces were developed, and three-sheets became two sections, six-sheets became 4 sections, and 24-sheets became 12 sections. It wasn't for many decades until a press was made that could print a three-sheet in one-section.
Almost no theaters had three-sheet or six-sheet frames. The vast majority of them were pasted or tacked or taped directly outdoors onto walls of buildings, which is why the bulk of them either survive in VERY "used condition" or in great condition (if they were unused). These posters were meant to be seen as you were driving or walking down the street from a good distance away, which is why the close detail is often not so carefully done (like where the sections matched) because one wouldn't notice such stuff from 50 or 100 feet away.