In general, for the first two slots you'll want to have the [+20% with T2 troops] item and the [+15% to all missions] item.
For the third slot, we recommend either the [+15% with T2 troops], or [+10 to all].
The [+10% to all] will pull ahead if you find you're not using the T2 troops fairly often.
Other options are the +40% to short (Elixir of Overwhelming Focus) and +40% to long (Vial of Timeless Breath) items which have more situational usefulness, but you'll often find yourself more limited in which champions to use for certain missions.
Legion Mage Glyphs
If you're lucky enough to get a legendary item with [+15% to all missions], definitely use that as a third item.