Here's a primer on the math of available combinations. However there are exceptions and the numbers get exponentially high afer 4 letters/characters. At 5 or 6 IDN combos kick in complicating the math even further in to algebra formulas.
The math of available combinations (ASCII, Not IDN):
26 x 26 = 676 (Two letters A-Z)
36 x 36 = 1,296 (Two Characters A-Z, 0-9)
10 x 10 x 10 = 1,000 (Three Numbers only, no hyphens)
26 x 26 x 26 = 17,576 (Three Letters A-Z, no hyphens)
36 x 36 x 36 = 46,656 (Three characters A-Z, 0-9, no hyphens)
36 x 37 x 36 = 47,952 (Three characters w/hyphens possible for middle position only)
26 x 26 x 26 x 26 = 456,976 (four Letters only A-Z)
26 x 26 x 26 x 26 x 26 = 118,813,376 (five LETTER ONLY combinations).
You can see how the possible combinations jumps from 3 to 4 letters only by going from 17,576 to 456,952. Odds of picking a 3 letter acrnoym that's usable are fairly good worldwide at 17k, Odds go down greatly at 4 letters, meaning at 4 you need real words.
The math above still does not account for reserved names by ICANN such as other TLD letter combos in the second level domain position are reserved combos for ICANN and the registry itself such as nic.tld,
www.tld, etc. Also, as you get to more characters, there are disallowed combinations such as two hyphens together that indicate IDN names, and some TLD's like the .us that reserve possible combinations of zip codes, phone numbers, and state codes.
In answer to the question, it's a matter of supply and demand. There is a high demand for TLA's (three letter acronyms),but a fairly limited supply on a world basis. TLA's can also be used as people's initials, and have the value of being very short to type as email addresses, even if harder to remember than real words. At 4+ letters, acronyms get very hard to remember. I'm in an organization that has a 5 letter acronym and new members never can get it right at first. Even so, The Domain Name AfterMarket chose tdnam.com as the short version of their name as a surprise to me, even though thedomainnameaftermarket.com also works.
Three word domains are valuable in .com solely because most good one and two word domains are taken based on the demand and size of the .com list of taken names and "default" nature of the .com TLD.