Said to have had a fourth daughter.

William's daughter, Eliza, had been equated with Elizabeth D. THOMPSON, who married George Alexander FLOOD in Trigg Co., KY, in 1853.  These are two different women.  Both show up in the 1850 Census of Trigg Co., Eliza with her father (see below) and Elizabeth D. with Miles and Sarah MALONE, probably as housekeeper.  Eliza was born 1834/5 in Kentucky; Elizabeth D. was born 1832/3 in Tennessee, probably the daughter of Mrs. Mary THOMPSON, a widow who also shows up in the 1850 Census of Trigg Co. 

1.  Jordan R. Dodd, ed.  1993.  Kentucky Marriages:  Early to 1850.  Liahona Research, Orem, UT (Broderbund CD-229):

Thompson, William C.

Daniel, Lucy

27 Sep 1827

Trigg Co.

2.  1830 Census (extracted by A. Robert Matthiesen; see 1830 Table):

1830

William C. Thompson

000 010 000 - 100 010 000

Cadiz, Trigg Co., KY

p. 9

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
1 male	20-29	1800-1810	= William (b. 1802/3)
1 female	4 or under	1825-1830	= Emily? (b. 1830)
1 female	20-29	1800-1810	= Lucy

3.  1840 Census.  William is said to be in Osceola, MO, but he does not show up in the census there (at the time, Rives Co., now St. Clair Co.), at least not as head-of-household.  The only Thompsons in Rives Co. in 1840 were Samuel and John.

4.  1850 Census Index/Microfilm (online at GenealogyLibrary.com): 1st Dist., Trigg Co., KY, p. 292, 33/33, enumerated 3 Aug 1850 (extracted by Diana Gale Matthiesen):

Wm. C. Thompson	47	M	Grocer	Virginia
Sarah Thompson	77	F		same
Robert W. Raine	24	M	Tailor	No Carolina
Emily Raine	19	F		Kentucky
Eliza Thompson	15	F		same
Lucy Thompson	13	F		same

Sarah is William's mother, Sarah (STEELE) THOMPSON.  Robert RAINE is William's son-in-law, husband of Emily.. 

5.  Barney Thompson. MS. The Thompson Family of Prince Edward County, Virginia, Butler and Trigg Counties, Kentucky, and the Florida Parishes of Louisiana:  the Descendants of Thomas Thompson, 1749-1810.  Copies at Western Kentucky University, the Filson Club Library, and the Indiana State Library, Indianapolis.  (Excerpts courtesy of the author.)

6.  Barney Thompson, ed.  1996.  "Letter to the Editor of the Kentucky Telephone, December 20, 1889."  Pages 6-10 in Pioneers of Trigg County, Kentucky, As Seen through the Biographical and Genealogical Articles of Cyrus Thompson in the Kentucky Telephone and the Cadiz Record, 1889-1899.  [MS] (online at the Trigg County KY GenWeb site).  The excerpt on our subject reads:

William C. Thompson was pretty much reared in Cadiz, and resided there until about 1835, when he removed to Missouri, and located at Osceola on the Osage River.  He was a saddler by trade, and for a number of years carried on the saddler's business in Cadiz in copartnership with Thomas S. Thompson, and he also worked at his trade when living in Missouri.  He was an unusually good looking young man, possibly the best looking in his young manhood of all the Thompson family, and was amiable, honorable and then moral.  He married in 1828 or 1830 Miss Lucy Daniel, a sister or niece of George Daniel, one of the first sheriffs of Trigg county, who was thought to have been one of the handsomest young ladies in the county at that time, and in her day.  His family accompanied him to Missouri, but about two years after going there his wife died, leaving four children, all daughters, and these he soon thereafter took back to Kentucky and to Cadiz and placed with my mother, who reared them.  He remained in Missouri until about 1843, when he returned to Cadiz and remained there until his death in about 1869.  He was a generous man and warmhearted young man, unobtrusive in his manners, but smartly lacking in energy, and was not, I think, a success in any line of business he ever pursued -- dying poor.

Cyrus' memory had faded slightly.  William and Lucy were married in 1827.  Their daughters were all born in KY, so they could not have gone to Missouri until after the birth of Lucy in 1836/7.  William does not show up in the 1840 Census of Rives Co. (county seat, Osceola).