I think you are looking for Mylor, The Most Powerful Horse in the World by Michael Maguire (1977).
"The book concerns a group of inventors racing to create a robot horse that can win the Grand National without being detected as a robot. At least one edition of the book features a cover illustration of a brown horse w/ white socks & white blaze on face, and three children, superimposed over a circuit/transistor board. The children are a blonde teen in a plaid jacket w/ white wooly collar, blonde girl in lavender sweater, and brown-haired boy w/ glasses in a brown/blue striped shirt. There is also another book, probably a sequel, called Mylor: The Kidnap. These books are pretty scarce, but can be found online through several U.K. booksellers, as they were published in London."