So from what I understand, merges are additive and only add 1 more lift point per merge. So a +3 merge gives 3 more lift per run.
Running multiple mythic heroes though is multiplicative.
There's a flat bonus of x10 to lift gain if you run any mythics at all, so with that in mind:
One +3 Eir and four light blessed units: 1x4x10 +3 = 43
Two Eirs, both at +1, and 3 light blessed heroes: 2?3?10 +2 = 62
And then add 80, the base lift gain, and 20, for the bonus unit, to those numbers to get your lift gain per battle.
Running three Eirs is only 'optimal' if they're all max merged, though I don't know how feasible that is in usage.
For your case, I'd it's best to just use two Eirs. Considering you only get a single point of lift per merge, you might even just save the extras for skill fodder.