If you use three different digits, you'd have 4 choices for the hundreds digit, 3 for the tens, and 2 for the units, so 4*3*2 choices in total. If you use two 1's and a different digit (2, 3 or 4), you have three choices for which other digit to use, and three choices for where to put it - the hundreds, tens or units place - so 3*3 numbers you can make with two 1's and something else. So you have 4*3*2 + 3*3 = 24 + 9 = 33 different numbers in total.
You will *never* see the number 33 written as "4P3+3C1*(3!/2!)" in a GMAT question - it's not only an absurd way to write a number as small as 33, but notation like nCr or nPr is not something GMAT test takers are required to know about for the test (in mathematics, there are about a dozen different notations that are used for those quantities - notation like nCr is not standard in mathematics, and many people will have studied a different notation, which is perfectly fine). So I'm curious about the source of the question; the answer choices are not GMAT-like.
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