the probability of your 3 cards to be all jack is 4 over 52.
WHy?
because in a card deck, there are a total of 52 cards and each value of of the cards has 4 copies each.
For the jack, we have 4 and here are the following
=> Jack of spades
=> jack of hearts
=> jack of diamond
=> and jack of clubs
Thus, the probability is 4 over 52.