Learn Russian: В тексте три ошибки. - There are three mistakes in the text.

After the numbers 2, 3, and 4 (like три here), Russian requires the noun to be in the Genitive singular case. Because ошибка (mistake) is a feminine noun ending in -а, its Genitive singular form is ошибки.

Why does the Genitive form of ошибка end in -и instead of the usual -ы?

This is due to a famous Russian spelling rule. Usually, feminine nouns ending in -а change to -ы in the Genitive singular. However, you can never write ы after the letter к (and six other specific consonants). Therefore, you must use -и instead, making it ошибки.

Where are the words for "there are" in this sentence?

Russian typically leaves out the verb "to be" in the present tense. To say "there is" or "there are" in a specific place, you just state the location first (В тексте meaning in the text) followed by what is there (три ошибки meaning three mistakes).

Why does текст have an -е added to the end of it?

It is in the Prepositional case because it follows the preposition в (in) to describe a location. For most masculine nouns like текст, you add an -е to the end to show where something is happening or located.