It depends. I myself not a big fan of three word combinations and trying to stick to two word gererics. However, it's all about quality of the combination. I have a few three word combinations which are doing pretty good such as "City"CheapHotels.com
Also, I see that some three word combinations go for 2-3k on auctoins e.g. MiamiBeachRestaurants.com HeyGoodLooking.com and many more.
I had a few 'personal' pretty long domains in the past including, 'duwuerdestmichnichtsuchenwenndumichnichtgefundenhaettest.com' (for the wedding of my brother) and 'eierschalensollbruchstellenverursacher.com' (for a personal experimental site/playground.).
I guess those sure would count as way too long, but still they were fun to explain to anyone.
It all depends on the domain. There is no number of letters that makes the domain too long. A three word name such as FreeCreditReport.com would be worth a fortune even though it has 16 letters and is 3 words. VisitLasVegas.com would be worth a fortune too and so would LasVegasRealEstate.com even though it is 18 letters. I would stick to 3 words or less though and keep it generic.
I think any domain that has a market is ok no matter the number of words. Depending if you're going for type ins that may make a difference. I own a couple of 3 worders and they get a steady flow of traffic/rev., it helps that they are generic and not randomwordorder.coms. Of course you could always go for one like this http://www.howmanyfiveyearoldscouldyoutakeinafight.com/ etc...
i've sold MANY three and four word domains for huge profits. it's important to have specific, keyword names and not add fluff words like BIG, SUPER or any other generic extra word just because the name you were looking for was taken.
Though I am not a fan of REAL long domain names, the choice of words is much more important than the number of letters in a domain. A three word common phrase is better than a two worder that makes no sense and may be shorter.
I think 3 words combinations are still accept to me though it a bit long. Those short& pronounceable LLLL/LLLLL.COMs are not easy to get now. A long buy correct keywors combinated Domain is easier to be remenbered than raNDOM LLLL in my mind, and keywords combinations work better on SEO than LLLL/LLLLs
e.g, I have the name californiamorgtagerefinance.com, it even easier to be remembered than "nxkqj.com" even it is 28 characters long