Exactly three aces: First pick the three aces out of four aces in the deck and then pick the remaning 4 cards from the remaining 48 cards. This, divided by the total number of possible selections of top 7 cards, gives the answer,

\[\frac{ {4 \choose 3} \cdot {48 \choose 4} }{52 \choose 7}\]

Exactly two kings: Just like the above,

\[\frac{ {4 \choose 2} \cdot {48 \choose 5} }{52 \choose 7}\]

Either or: You need to sum the above probabilities and subtract the probability that both events occur at the same time, which is \(\frac{ {4 \choose 3}\cdot {4 \choose 2}\cdot {44 \choose 2} }{52 \choose 7}\).