1. Change the numbers into the Arabic numerals.

2. Change the numerals into the Arabic numerals.

3. Write the Arabic numbers in English.
(Are they all grammatical?)

They are all grammatical, but omit "the" before Arabic in #1 and 2.

1. To change numbers into Arabic numerals, you simply replace the symbols or words representing numbers with the corresponding Arabic numerals. Arabic numerals are the numeric system most commonly used in the world, consisting of the digits 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. For example, to change the Roman numeral "XVII" into Arabic numerals, you would write "17".

2. It seems there might be an error with this question. The term "numerals" usually refers to symbols or words representing numbers. So, changing "numerals" into Arabic numerals would be redundant since Arabic numerals are already the most commonly used symbols for numbers. If you meant to ask how to convert Arabic numerals into a different numeral system such as Roman numerals, please let me know.

3. To write Arabic numbers in English, you simply express the numerals as words. For example, "1" would be written as "one", "2" as "two", "10" as "ten", and so on. This method is often used when writing out a number in textual form in order to enhance readability. However, when dealing with numerical data or in mathematical contexts, it is more common to use Arabic numerals rather than writing the numbers out in words.

Regarding the grammaticality of the three statements, only the first and third statements are grammatically correct. The second statement mistakenly asks to change numerals into the same Arabic numerals that are already being referred to.