Actually, there is a problem with Gary''s Student's formula (assuming I
understand your request correctly)... it only works if there are exactly 3
digits in front of the decimal point. This formula should what I think you
want under all possible circumstances...
=TEXT(A1,"0\:00.0")
but note that if you have less than 3 digits in front of the decimal point,
it put a "0:" in front of the number (which is formatted out to have a
leading zero where necessary).