Good Dictionary words could fetch high $xxxxx, where as just LLLL's without any meaning may be worthless for few and priceless for those who can use the abbreviation for their business or company.
LLL.com from high $xxx up to millions LLL.net may worth just low $xxx if the're no special meaning on the name or abbreviation.
no of letters in new domain registration not matters but if u want to buy existing domains, then catchy and big names are in millions/billions, it depends on many factors, how catchy domain is, how old, backlinks, traffic. You are out of luck because domain buying selling is a big industry and finding cool domain of 2/3 letters in almost any niche is hard, ppl register millions of domains and park them for profit.
3 letter domain - You can check the pricing guide here 4 letter domain - Depends on the name itself. The top sales LLLL.com is Fund.com sold for $10 million last year. But if you have crap LLLL.com like qzxq.com, then the price is around $10.