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100 five letter words begin with IN, and almost all of them are the prefix attached to a three letter root.
D leads the third box at about seventeen percent, then C at nine, K and T at eight and S at seven.
INDEX, INPUT, INNER, INTER and INTRO are the most common entries, and they cover four different third letters.
INDEX, INPUT, INLET and INCUR share only those two opening letters. Nothing else about them lines up, so confirming IN narrows far less than a normal two letter match.
That is why the third box matters more here than the prefix does, and why a guess should aim at D or C rather than at the shape itself.
INDEX is the sensible opener. It covers the leading branch and its E and X settle a great deal, since X appears in almost nothing else here.
It identifies a prefix rather than a word shape. The three boxes after it can hold almost any workable root.
It repeats both its N and its E, so it tests only three distinct letters. INDEX or INLET rules out considerably more.
A noticeable share are, alongside country and place names. Treat the count as generous rather than as a list of playable options.