Learn English – Are all officially recognized Scrabble words English

There is no word with three consecutive letters under the most narrow definition of "real word", but there are several words of the following types:

1. There are many acronyms/initialisms that contain triple letters, like AAA, AIEEE, KKK, WWW, or Roman numerals like xviii.

2. There are place names, like

   • Kaaawa, a place in Hawaii.
   • Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch, a place in Wales.

3. There are names of people with all sorts of unconventional spellings, like Minnnie, Annnora, or Diannne.

4. There are interjections like mmm, hmmm, zzz, etc.

5. There are archaic/poetic words with elided letters, like whenceeer (really, whencee'er from whenceever), wheree'er, etc.

6. Finally, there are words which would probably be written with a hyphen whenever used, like frillless (mentioned by Dusty) which is really frill-less, similarly bulllike, gulllike, wallless, many words ending in -ss-ship like your princessship, bossship, goddessship, headmistressship, patronessship.

The latter: it's not a standard word that you'll find in any dictionary. The suffix -athon, as you've guessed, comes from marathon, and means "an event, as a sale or contest, drawn out to unusual length, often until a prearranged goal, as the contribution of a certain amount of money, is reached" (according to Dictionary.com).