The probability(s) in grade(s) 1, 2, 3, is (are) 0.1,0.2,0.3, respectively.
Should I use singular or plural for those parentheses?
In this sentence, one should use "are". Three separate grades, each with its own probability figure, are given so a plural form is proper.
I would also modify the opening of the sentence to:
The probabilities in grades 1, 2, and 3 are ...
and I might spell out "one, two, and three". I would also put spaces between the figures, as "0.1, 0.2, and 0.3".