u/Einheerjarr

Item 1

Crit damage multiplier
Maximum life
Intelligence
Weapon damage

Item 2

Crit damage multiplier
Maximum life
Life on hit
Cooldown reduction

Item 3

Crit damage multiplier
Life on kill
Resource cost reduction
Vulnerable damage multiplier

Pre roll existing affixes in combining new item from highest to lowest where affix appears at least twice (I.e, in 2 items)

Pre-roll 1: (number of rolls: 4)
Crit damage multiplier 3/12

Pre-roll 2: (number of rolls: 3)
Note the number of rolls may be 4 if you were unlucky in a previous pre-roll.
Maximum life 2/12

If an item has multiple affixes that appears 2 or 3 times, then the order the affix is pre-rolled is randomly selected.

Lastly, apply randomness in selecting the remaining affixes. Select these from affixes that appeared at least once:
(Number of rolls: 2)

Intelligence 1/12
Weapon damage 1/12
Life on hit 1/12
Cooldown reduction 1/12
Life on kill 1/12
Resource cost reduction 1/12
Vulnerable damage multiplier 1/12

This will make 3 to 1 recipe more targetable by recycling items with similar affixes (e.g., ensure intelligence is in all 3 items you are recycling)

You will essentially have 4x 1/4 chance to keep your crit damage multiplier by ensuring all 3 items you included in the cube have critical damage multiplier while targeting other affixes you desire either via randomness or by having the affix to appear at least twice within the 3 items you are recycling.