I cant seem to find this info anywhere, if i have a 5x3 gun and i hit an enemy with 15 damage threshold, does it land or not?

No. The 5x3 means that you're firing 3 projectiles which each do 5 damage. So that 15 DT would block the shot

No DT will completely negate 100% of the damage applied from any weapon. This is the same for the player and the enemy. No matter the DT, 20% of the damage will still be inflicted. If your weapon fires 3 projectiles at 5 damage each, then each projectile is calculated separately. With a DT of 5 or greater, the damage done would be 3 total. Providing all three projectiles landed. 20% of 5 is 1, x3 hits.

EDIT : That is unless the "X #", you are referring to, is the critical chance of the weapon. But I'm sure you are referring to the display of the weapon damage in the pip-boy menu.

oh, thanks for clearing that up.