Statistically, four out of five randomly rolled characters will be better than the standard array, and one out of five will be worse.

The chance for a given highest-three-of-4d6 roll to be lower than 15 is 76.85%. So the chance of all six of them to be lower is 76.85⁶%, that is to say about 20%.

For hitpoints, on the other hand, getting the standard is often much better, around six percent better on average. Rolling for hitpoints is a serious gamble. But who know, you might get lucky.

It's up to you whether to do standard array or rolled abilities, and every level you can choose whether to roll you hp or get the standard.

(The so-called "point-buy" option is forbidden, verboten, banned, behated etc etc. Pox upon it!)