There is only one way of getting 10, J, Q, K in one suit and there are 4 suits, i.e. 4 ways of getting the required cards. There are 48C3 ways of picking the other 3 cards. There are 52C7 equally likely ways of picking 7 cards from 52. The probability you want is 4 * 48C3 / 52C7. nCr is the binomial coefficient of picking r things from n.