Example:
Your mech has 3x Damage 30%. In a successful hit for 50 damage, your 30 % chance to activate the double damage works, dealing instead 150 damage (50 x 3) to the enemy Mech.
Combos:
If you got both, 2x Damage and 3x Damage activated, you may deal 300 damage instead with that starting 50 dmg hit (x2 and x3 will produce x6 damage if both chances worked).
Tips:
It's a good idea to use equipment and weapons to increase damage. "There is no better defense than a good attack".
Focus on a few skills you want to enhance your mech and use as many pieces of equipment with such skills as possible. Also consider that a weapon with a rated damage lesser than another, may be better if included a high percentage of increase. Especially if your mech already have a high possibility of tripling the damage.
Notes:
Be aware that the mech can get this skill included in its build, as equipment or in weapons. If the skill was included as a Mech build skill or into the equipment, every weapon will have a chance to produce double damage. But if the mech has not the skill nor the equipment, only when using those weapons with the skill may be a chance to get this effect.