What Are The Next 3 Letters ?

What are the next 3 letters in this riddle? 

O T T F F S S _ _ _

The sequence of letters "O, T, T, F, F, S, S, ?, ?, ?" is actually a sequence of the first letters of the written-out numbers from one to ten. Each letter represents the first letter of the word for each number.

For example, 

• The first letter "O" represents the first letter of the word "one". 
• The second letter "T" represents the first letter of the word "two". 
• The third letter "T" represents the first letter of the word "three". 
• The fourth letter "F" represents the first letter of the word "four". 
• The fifth letter "F" represents the first letter of the word "five".
• The sixth letter "S" represents the first letter of the word "six".
• The seventh letter "S" represents the first letter of the word "seven".

The question asks for the next three letters in the sequence, which are "?" "?" "?". To find the next letters, we need to continue the sequence of the first letters of the written-out numbers from eight to ten. The eighth letter is "E", which represents the first letter of the word "eight". 

The ninth letter is "N", which represents the first letter of the word "nine". And the tenth letter is "T", which represents the first letter of the word "ten".

So the complete sequence is "O, T, T, F, F, S, S, E, N, T".